So far this version, and I think last version, no tomb under town chapels have had waking mummies, only in tombs away from settlements. One way to tell if a mummy will wake is if the loot is markedly better than anything else you find in the region. It's schmuck bait; you'll always find something irresistible to pick up. Not picking up anything is probably a wise suggestion if you're not tomb
raiding -- otherwise, wtf is the point of even going in? Just on tour or something?
It's best to get a disposable companion to walk beside the tile while you go for relative safety, or at least a position you're not surrounded. After casting it's curse, the mummy isn't any more dangerous than another necromancer.
Max's suggestion to save/reload isn't foolproof; if you triggered the mummy but evaded the curse, the mummy may stop hunting you, but if you stumble across him and he has a line of sight on you, no matter how distant, you will get hit. I found this last version when I absconded with my loot, then bumped into him while trying to clear a necromancer's raiding zombies from a hamlet.
So out of two libraries in this world so far, the awful oubliette with no room and like a hundred people packed into less than a dozen squares was utterly peaceful... the big open elf library with no walls at all is full of blood and people stabbing each other in the head.
How do you locate a library? Is it displayed on the map in some color? Is there a sign?