Why the bathroom
Silverfish need humidity and they feed on starch: wallpaper paste, book glue, cardboard, the sizing in fabrics. A bathroom offers the damp, and the airing cupboard, skirting gaps and the space behind the bath panel offer the shelter.
What they damage
They rasp the surface of paper and card, leaving irregular holes and a yellowish stain. Photographs, books stored in the bathroom cabinet, and wallpaper edges are the usual casualties. The damage is slow, which is why it is often found late.
A few or a population
Sightings are a poor guide because silverfish are nocturnal and fast. A monitor on the skirting draws them with a lure made for their group and counts them as they enter. A low, steady number is a few wanderers. A rising number is a breeding site, and the fix is to find and dry it out.