Found this 6mm long orangy/brown hairless grub under rugs laid on carpets. The grubs could only have been there 4 days as there was no sign of the at weekend. I guess under each rug there were 100 grubs ( 3 separate rugs), the vacuum cleaner got them before I was allowed to count!
It's possible they are from a Bluebottle fly and the maggot was feeding on something dead in the vicinity. Possibly a dead bird in your chimney. The maggots drop off the meat and seek out a dark place to pupate and metamorphose into a fly.