I have a lot of big orange slugs in my garden (plus brown ones and horrrible snotty-looking green ones), but not round ones -except when one has curled itself into a circle.
When we first moved here, all our slugs where the traditional large, black ones, but they have all gone!
Luckily, none of the slugs have ever found their way into our house, but DS1's first house seemed full of them - the day after they moved in, they went downstairs to see slimy silver trails all over the living room floor. DS eventually traced these back to one corner of the room, where some wires came through the wall (near floor level).
I think he filled any visible gaps but also boxed the area in, then put slug pellets in the box.
It solved his problem, but if you don't have a clear enough trail to follow back to the entry point, I'm not sure what to suggest ?