Years ago I had a housemate and she told me that every night, a slug would leave a trail on her bedroom mat and this was going on for years. She could never find it or catch it.

She went away for a week or so for work so I poured some salt on her mat.

Next morning I go look and I find the slug stuck in the middle of the mat unable to escape.

Took a photo of it, tossed it out into the garden and vacuumed the mat.

Showed her the photo when she returned and told her what I had done. She was so relieved that the phantom slug had been dealt with.