A pack of stray dogs have taken part of the mine as their lair (or someone's using it to breed something like Rotweilers for nefarious purposes).
Someone might have moved something like a drug lab into it, especially if it's fairly remote, but there's a large city like Swansea or Cardiff not too far away (particularly those two as you then have road and rail links into English cities like Bristol and then through the Thames Valley into London, although North Wales gives you fairly good access to Liverpool, Manchester and Birmingham).
For a non-combat encounter, local kids could use it for underage smoking, drinking etc.
A flock of bats, or an owl's roosting in there, or some other nocturnal animal.