I had the same problem in a house I lived in.
I think they were getting in through an air brick. They can squash themselves flat and get through quite small gaps.
Go round the outside of your house looking for gaps like air bricks, gaps between frames and doors etc, anywhere else they might be able to get in and close the gaps.
Don't block air vents though - you will get damp. Put some fine netting over or (like I did) pile some gravel up.
This sorted it for me. A much nicer place to live when you don't have to do "slug patrol" first thing in the morning...
