Gregorius will have a go at casting a spell to detect spun vis in the cave temple, and inspect the mosaic and the altar more carefully, on the offchance that the artifact is one of them rather than something removed from the room.
[Spell is Base 1 + 1 Conc + 3 Hearing = 5. Casting total (In 6 + Vi 5 + Sta 1 + Aura 5 + die 4) /2 = 10.5, so succeeds. [Coyote Code doesn't seem to be working properly for me, so have just rolled a die manually.]
He'll also question Theodoric as to whether he remembers anything in particular regarding the cave.
The regio, however, gets his interest more keenly. He's fairly sure given the strength of the magic aura that if he'd been looking at a magic regio he'd have spotted it. [Second Sight would have been Per 3 + Second Sight 3 + Aura 7 + die =13 + die, whereas the Ease Factor to see into even a level 10 regio would have been 5 + 2 x (10 - 7) =11.] That leaves a couple of possibilities. Either the regio level wasn't magic (which seems highly unlikely, but he's not actually certain is impossible), or he wasn't looking in the right place.
He'll ask Viola to do a few castings of Piercing the Faerie Veil just to cover off the first possibility (and then just sighs resignedly when she makes it clear she thinks he's an idiot for thinking you might have a faerie regio above a magic one, and tells her he just wants to eliminate it). For the second possibility, there are a few places he could have not just not looked at - at some random spot in the tunnels, or even inside a wall - but the most likely place seems to be somewhere within the lake. That brings finding some practical way to explore the it without getting eaten by its inhabitants up the agenda.