What bug is this? I found several in my flat and I have no idea
That is the larva of a carpet or skin beetle, family Dermestidae.https://en.wikipedia.org/wiki/Dermestidae
These eat dried flesh, skin, and other dried organic matter. They are the bane of entomologists because they eat insect collections. They are used by bone-collectors to clean skeletons, and are the last insects to arrive at a corpse.