Orry, I appreciate your conundrum.
Unfortunately, the devices used in vending machines are generally tilt/mercury switches, which aren't versatile enough for this application.
Mercury switch - Wikipedia, the free encyclopedia
Small, low powered accelerometers could be used (cheap versions of what's in an IRS), although I'm not sure of their availability, size, price or power consumption.
Thinking out of the box, how about something that detects transceiver squawk.
Transponder (aviation) - Wikipedia, the free encyclopedia
Very few aircraft don't carry these devices. They're pretty much mandatory in anything used commercially. Aircraft only squawk "mode A" or "mode C" when airborne, so the device could work like this:
- Can I detect a squawk (am I near an aircraft)
- Can I detect the same squawk a minute later (am I near the same aircraft, or is this a different one and I'm therefore just parked near a runway)
- How strong is the signal (is it the same aircraft, but it's just flying around above my house)