I've been getting exactly the same problem this year, including one particularly huge adult early in March that could in no way have been a new Spring hatchling. I think these loners have crawled off into a corner last Autumn and hibernated, and are then waking up in the spring/summer, but with no food or moisture they don't last long and expire on the carpet.
The large wasps you find are usually queens found early in the year, dead wasps are sometimes found to have died from natural causes, or overheated from flying around windows and light bulbs, if you are seeing many, as Archbishop has suggested, you may have a nest in your loft, a bit eary in year for that though. from a pest controler