WarmGlass.com

I took a mold to a nearby studio where other people work. It was there for a long time (studio had closed for COVID in the meantime). I found out that other people had been using It before I picked it up.

I'd like to write on 'please don't use without permission' or something along those lines on the top side so it's apparent to others, but don't want it to transfer to the glass. I normally use ceramic pencil to put my contact info on the bottom. Would that transfer? Would a regular pencil work?

I wouldn't do anything to the glass-contact surface. You could use anything on the bottom but that's assuming that people would look there before using the mold.
How about hiding them? Putting them in a bag or box that says "keep out!"? Taking them home with you? Or having a meeting and sharing your concerns, perhaps setting up a mold-sharing protocol?