What a near-perfect-accuracy yet dirt-cheap way for a drone or other miniature embedded computer to sense whether it's indoors vs outdoors? Some ideas:
- Geo position sensor if small, low power, and cheap enough
- Light sensor to detect the 60 Hz flicker of electric lights
- Antenna and analog filter to detect EMI typical of occupied buildings
- Echo generator & microphone
- broad-spectrum UV-visible-IR photosensors to discriminate natural vs solar light
- Thermometer, barometer, or other air sensors