I always assumed the timer started as soon as was no longer held because they didn't blow up in inventory
Well, you can change them to blow up in the inventory, too, if you wish.
Mines are basically heavier proxies. They trigger when someone move into one of the 8 adjacent tiles - but not when someone walks under them or hovers over them, if you're into that. Or onto the tile directly, but that's largely limited to fliers and elevators.
If mines seem too heavy, proxies are some ways up the tech tree. There are even improved proxies in XCF.
Mines are very much
supposed to lurk and surprise you.
There can even be mines that are invisible on the minimap (and/or have no floorobj), but thankfully these haven't migrated over from Piratez. Well, except for one little surprise.
No way to hide walls that I know of. X-Com was before that became a thing.