Have you a nest in the loft or hole they could be getting in and then getting in through a loose light fitting or a cracked open window. Pop outside and see if you can see any entering you house.
The other suggestion is because of the strange weather weve had, you may have had a some in your house last season and they are coming out of hybernation later than usual. Workers normally die off though through the winter but could survive in a warm house.
The fact you find them dead by the window is because they can see daylight and trying to get out!