Item for discussion on the 727. The APU was in the main wheel well. The exhaust, if you could believe it, was on top of the wing just about next to the overwing exits.
If the APU ingested snow or de-ice fluid, it would torch when it started. Not a big deal in the day time, but at night it could be bright. People would see it and freak, and bingo, unguided tour of the top of the wing.
Richman