Hi, Steve!

Do you have a picture of the insects you're finding dead in the garage attic space?

The deposits on the siding could indeed be sign from bats roosting between the planks. If there are bats roosting in those spaces, you may not get a good sighting to confirm (evening exit to hunt) until a relatively warm evening occurs. We generally advise not sealing/excluding bats from roosting spaces until fall (after young can fly on their own and exit), but there's a chance you've got adults/subadults using your siding as winter roosts, which could complicate things a bit - as I mentioned it might be harder to confirm. Perhaps try this - Clean off the marks now, and see over a week or more whether and specifically where they reappear, if at all. It might be that the marks are from spring-summer roosting and that the bats are gone so you could go ahead and block those spaces, but you definitely don't want to do that if you're trapping an animal in there! I have no knowledge of blow flies - We'll need another expert to help with that part of your question.