So I rammed a lot more coarse steel wool into that hole then put a whole tube of that sealant into it. I didn't notice that there was a protective cap on the bottom of the tube & squeezed the caulking gun so hard I bent the handle, making it useless. I ended up cutting the top off the tube, pushing the gunge out & pushing it into place with a spatula.
There's a 2nd large hole where the kitchen sink waste goes through the wall. I've put more steel wool into there but would need to take the back of the unit off for access to do anything else.
I'll be a lot happier if I can keep them out of the kitchen. It's the only room we know they've been in, and the health implications for 2 elderly people, one with a compromised immune system are not pleasing.
Guess I'll have to check the bathroom pipes as well. My sister who lives with them has read that they can swim out of a toilet & is now insisting that lids be left down at all times !