Isen will go.
After his discussion with Fiona, he is eager (so to speak) to meet the blacksmith, so he might as well kill 2 birds with one stone.
Unfortunately, using a fire drake as an Arcane Connection might be frowned upon, by the drake....
Okay, time to bring out the big guns here (or at least pretend to be a big gun). Tranquillina will go to the covenant's ceremonial casting room and spend five minutes sponting (hopefully) the spell Sight of the Molting Magus from HoH:TL, page 72: it causes Corpus material to glow red to her vision. She'll use exaggerated words and loud gestures (or the other way around). Here we go: Sta -3 + In 6 + Co 27 + aura 5 + words/gestures 2 + casting room 5 + Talisman attunement (eyes) 6 + Artes Liberales 1 + stress die 3 = 52 / 2 = 26 - made it.
She'll then go to wherever Marcus was staying and start looking around for hairs, fingernails, stray bits of poo, whatever. Oh hell, walking means a Concentration roll, which is based on Sta, not Int. 
Well ... Sta -3 + Concentration 1 + stress die 5 = 3, just barely. (The juxtaposition of how hard the last two rolls are for Tranquillina really says a lot about her, eh?) Even if there's a sanctum marker, she'll ... gulp ... go on in (after calling his name a few times, just in case) - but not to the point of crossing any Circle-looking wardy things (not that that seems likely).
Assuming she finds something, she'll gather up whatever she finds and go back to the casting room, to double-check that it's from Marcus - that's The Whole from the Part from the same HoH:TL page. She doesn't need to scream this time: she gets Sta -3 + In 6 + Co 27 + aura 5 + casting room 5 + Talisman attunement (Corpus) 3 + Artes Liberales 1 + stress die 2 = 46 / 2 = 23.
Hopefully this results in a verified Arcane Connection to Marcus; if so, she'll have other ideas for how to use it before they depart. Either way she'll report back to the others, and she's kind of tempted to accompany them.
As a familiar, the Fire Drake has an Int score and all the languages that Marcus knew when he bound it. This presumably includes Latin. The PeneAid from the Familiar (+4) is way higher than from that bit of skin and hair(+1) you found.
a) It's a spont, so make it Diameter instead of Concentration. Then you can Leap to the door of Marcus's room.
b) It's a 5th-magnitude spell, meaning it takes 15 minutes * 5 = 75 minutes to do it, not 5.
c) If we're willing to wait a day, and the Fire Drake was willing to provide us with his birth details, then one person could do his nativity horoscope and another person could do his daily horoscope, while whoever is casting the spell can spend it building a symbolic representation of him -- +5 to the multiplier.
d) If the Fire Drake really trusts us, then he might also be able to provide any Names that Marcus has we don't know about, for another +2, bringing us up to a x12 multiplier.
Alexei and Fiona have the highest Penetration, 3. Alexei's Penetration specialty is Corpus, which would get us to a base Penetration of 48.
"Now. Who has Wizard's Communion memorized by now?" asks Viscaria, seizing upon Traquillina's Initiative.
Hmm, reasoning with it. Never thought of that.... If someone wants to go find it...!
A better idea in hindsight. 
Aha! HoH:S, pages 59-60, tell us that "prepared spaces" for ceremonial casting reduce the time as well. I'm pretty sure Mons Electi's room qualifies as the "one minute per magnitude" variety at the end. That makes the room worth it even if there were no Casting Total bonus!
I think that the horoscopes also have to be done by the person casting the spell. I know it's more explicit for the symbolic representation (ArM5 page 84), but it does say "Caster" for the horoscopes. Unless it means "caster of the horoscope"? But "Caster" is used in the blood relative line and clearly means "caster of the spell" there too.
Tranquillina reluctantly raises her hand in response.
Korvin raises his hand.
[color=blue]"Who among us has the best chance of surviving a fire-drake hunt?" Fiona asks with a smile.
As does Fiona.
"Well, I've met him before. On that island with the vampires. I can go looking for him."
We've got a problem. IIRC, arcane connections fade when crossing over the magic realm. Of course, our characters don't know that yet 
Isen frowns
Not yet, sadly. (He learns it in 1225)
(BTW, WTF?? Someone changed some of the background colors for some seasons of Isen in the Magi Planner - Why? - And I can't change it back)
I'd bet on Alexei, with me second, maybe.
((I think it's that arcane connections don't reach across regio boundaries. If you're in a regio with Magic 5 (i.e. the regio side to the covenant) and try to use an Arcane Connection to Leap of the Homecoming to somewhere in Autun, it won't work. Or vice versa. I think.
((I noticed that this weekend...I suspect it's a function of the AutoComplete, because it did that to me when I was entering Learn Greek from the Teacher". I just fixed one of my colored backgrounds by clicking on the field I wanted cleared, click on "Format" at the top of the page, and then select "Clear formatting".))
Not in this saga. Some can Leap into the regio, if they have an AC. Pretty sure I've established this.
I sit corrected.
Fiona will be doing her Miracle Max impersonation. She'll get them into the regio (and show them how to get in and out), then she will wave them goodbye with a heartfelt "Bye-bye, boys! Have fun storming the castle!"
She will be sorely tempted to stay and dally with the Blacksmith, but she's already wasted, what, three weeks (outside time) here? One more night/week would put a serious dent in her studies for the season...although it might almost be worth it.
I'm nearly ready to start a new thread (someone else could too). But Jonathan, can you give a ruling on whether Tranquillina does find an Arcane Connection to Marcus, even before we try the Where's My iDrake app? Her plan is to use The Eye of the Sage and The Ear for Distant Voices to gather information about Marcus's surroundings. (In theory she could try The Inexorable Search too - who has a map of the Magic Realm? Nobody? 
) Best timing: just after sunrise or sunset, before he has a chance to renew his Parma. And yeah yeah, magical scrying.... whatevs, we're all friends here.
Tranquillina is definitely willing to go along on the rescue mission once it commences.
Marcus's quarters do have a sanctum marker on them. So going in is a clear violation of his sanctum. Enter at your own risk. There's also nothing in the Covenant to state that this is a violation of the Protected Guest status.
Hmmkay, Tranqillina will instead wait until we see if the fire drake plan works out before crossing a sanctum marker. I don't know how high the odds would be that Marcus has rigged up his temporary sanctum with traps - but if he had, I gather they would be traps of the deadly variety 
(PS: I think it's resonable for a Protected Guest who spends at least a season at a covenant to claim a sanctum there - although they'd probably have to relinquish any other sanctum to do so. Certainly that detail would have been discussed when Marcus first arrived. Above Lina's pay grade. 
)
[color=blue]"It sounds like we have a consensus that we should go back to Magic and try to rescue Marcus," Fiona says. [color=blue]"Anyone who wants to go, meet me outside Apollodorus's sanctum an hour after sunrise tomorrow."
Isen will be here.
10 xp for Fiona
7 for Wilhelm
5 for each of the grogs.
Am I missing anyone?
Any particular rules for Adventure xp in this saga, other than being pertinent to the story?
Falko: Awareness 1 xp, Great Weapon 2 xp, Music 1 xp, Survival 1 xp
Jehanne: Awareness 1 xp, Single Weapon 2 xp, Latin 1 xp, Survival 1 xp
Nope, it's all as per RAW.
Legitimate off-scene activities can use XP, too. So, if you can come up with a believable [strike]excuse[/strike]reason that the character would have used that Ability, I'll probably roll with it...
Fiona: 5 xp Leadership, 2 xp Creo, 3 xp Herbam.
Wilhelm: 3 points Latin, 4 points Hunt