Puzzled, I can’t work this out

[Giddypants]

For over a month most morning s I’ve come into the living room and found what I think is either a slug or snail trail on my rug.
It has a start and an end but it doesn’t lead anywhere.
I’ve gutted the living room a few times and found nothing.
Where is it coming from?
The room is clean (usually untidy with the kids toys etc)
What could be causing it and how can I catch it?

Is this a ground floor room? Do you have a garden?

Yes, so the room has 2 french doors onto the back garden.

Have you lived in a place with a garden before? I've lived in loads of places with gardens and TBH I think it just always happens. I get that you're mystified by the lack of trail to the source but I've never looked that closely - I just hoover them up. I think it just comes with the territory of having a garden and especially if you have level-ish access from garden to house, and especially in wet weather. They just like to come indoors overnight - I can't see you stopping them!