bobad, people commonly find BSF larvae in worm bins and compost piles. I haven't noticed my maggots tumbling, but they are definitely shy.
I bet you're mistaken about not having BSF around you. The nice thing is that you rarely notice them because they don't have a reason to land on you and they seldom enter houses. All they want to do is find your ripest garbage, lay some eggs, and then they're gone. A few months ago I would have said I'd never seen a BSF. Now I know that all I need to do is to put out some smelly trash and the BSF will find me. Next time you have some smelly garbage watch what's coming to it and I'll bet you see some BSF.
In the old days in the South they were called "privy flies" because they commonly lived in outhouses.
edit: bobad, I just realized that you said you get 4 or 5 BSF in your house per year. That number in your house should indicate a very healthy population in your area.
