Last night the Troupe and I got into a discussion on lab security.
Now I pose these questions to you:
How would you mark a person? (Mark must be lasting and detectable via magic. Painless creation a priority but not a necessity).
How does your magus protect his/her sanctum?
Let's see :
ReAn(Co) : mark the person with a tattoo from animal pigments (variants with Herbam or Terram, depending on the pigment). Permanent. If you do it under the hair, it might go undetected for years.
ReAn : splash the target with animal ink (variants with Herbam or Terram, depending on the ink). Lasts until washed.
PeCo : creates a specific minor blemish. Permanent unless and until it heals naturally.
CrVi : "create a magical shell that gives false information about the target to Intelligo spells". Just choose a very outlandish information and a very low level (so most intelligo spells will miss it entirely). You'll need a ritual to make it permanent.
MuVi: you'll probably need to so some original research for this one: modify the target's sigil to incorporate an additional mark.
Someone suggested inserting a bit of gold under the skin.
This should be detectable using InTe.
Painless? Maybe not so much though.