That makes sense if it's old mines. There's always be the hope of coming away with something of value. I can't imagine that an old tunnel would provide the same incentive. The only reason to use an old tunnel the way I see it is to engage in illegal activity, like breaking into a building without being seen. Well, that could provide something of value also, but it just doesn't sound that innocent.
I agree mostly with you. However, I knew of some folks who used abandonded missile sites for D&D parties.
Honestly, I don't know the motivation and I certainly DO NOT like the idea of a vial being taken.