We have these larvae coming out of our ceiling and they are tiny with a maximun size of 1mm. The one in the pic is 1mm long. They started to appear a just over a week ago in the warm weather and we have had about 400 of them.
Not a beetle but a 'Bristly millipede', Polxenus lagurus, and my favourite millipede. They are occasionally found in homes in numbers crawling the walls, probably because the outside walls provide good grazing for them - they feed on algae and are harmless to you and household furnishings etc.