There's a mouse who appears to be finding his way into the stud wall that separates two rooms and scratching around in there overnight. There's no visible signs of entry or exit in that wall, I've not seen any mouse markings on cables or skirting boards, or mouse poo anywhere, so I suspect he's coming up from below via a gap in the floorboards and directly into the void in the wall.
As a first defence, back in February I drilled a bunch of holes in the wall and busted two cans of expanding foam into the void. That seemed to work and there was silence for a couple of months, but now he's back and noisier than ever.
What's the best course of action to eliminate him?
1. Put live traps down, however, I think he's probably living between me and my downstairs neighbours so they might not achieve anything. Maybe worth trying anyway.
2. Open up the wall for preventative measures. It's a stud/plasterboard wall so I could saw open a few inspection holes, or remove the bottom 30cm of plasterboard across the wall (the wall is about 2.4m wide) to see where he's coming in and line with wire mesh to prevent him coming up into the wall.
3. Scorched earth approach. Set fire to the wall.