I have had a problem with slugs now for years, every time I go into our kitchen I find a slug. If I go into the kitchen late at night I will probably find a few.
About 4 years ago I ripped the kitchen back to the walls as I assumed there were breaches in the walls but it was all dry, complete and tidy.
I have since stripped the kitchen out again, in case I missed something, and replaced the window, done some pointing outside, I can't see how they are getting in. There is no door in the kitchen, can they possible squeeze through the closed UPVC window?
They might be getting in through the French doors in the living room and slithering across, but then surely i'd see more of them in the living room, where I only rarely see them.
We have wood floors with a space underneath, could they be living/ breeding under there?
They are doing my tree in, next job is to put some copper strip around the outside of the house, hopefully that will be effective, until the pikeys nick it.