Yeah, as mentioned they are a carpet beetle and they eat organic fibres. Normally you will find them in your clothes drawers and wardrobe. Also, keep a look out for the larvae, they are like little maggots (
LINK)
Once you have them in any numbers, they migrate and will get everywhere. We had an infestation in the last house we lived in (rented). The larvae were in all of our clothes drawers, in the carpets, in our bed and everywhere. One thing that didn't help was having laminate flooring that has that organic fibre underlay. They prefer natural fibres like wool, silk and animal hair, but I was told they will chow down on any organic fibres - cotton being one.
The reason you find them on windowsills and window reveals is that this is the time of year when they hatch into beetles. They like to find light and warmth.
The main thing to do, if you feel you have an infestation, is to locate the source. An old birds nest in the loft or wall cavity vent, or accumulation of pet hair behind a sofa, or under the bed, or any number of other places (such as a dead animal somewhere where the larvae are feeding off the fur/feathers).
There are some studies that show the hairs from the larvae can lodge in the skin and cause skin reactions and irritation that mimic insect bites, and also irritation of the eyes. It apparently depends on if a person is sensitive to their hairs or not (like a kind of allergy).
Cheers