It's probably a slug trail, the little blighters can get through the tiniest gap so -
first off you need to check all your door and window seals, and block any that have even the tiniest gap. You must make sure your external doors are properly sealed all round, even the tops - they do climb, and will get in this way.
get some slub pellets and sprinkle your garden with them.
Lace all your paths and steps with salt at night, repeat every night for a few weeks.
If the problem persists, sprinke salt along all the room borders, especially over doorways and windowsills, that should at least deter them.
Good luck.