[0_____0]

I've been buzzed by a flight of military helicopters in the New Mexico desert. Not intentionally, they just happened to overfly my tent, and I just happened to have cell service somehow. I checked ADSB and sure enough they were flying dark.

[ceejayoz]

Not necessarily; the same remoteness that made cell signal sparse likely makes ADS-B ground stations unlikely. There has to be one in range for it to show up places like FlightAware. Plenty of dead spots; you can help expand the network! https://www.flightaware.com/adsb/piaware/build/

[FireBeyond]

I have an ADS-B receiver on a computer here, and am overhead a number of flight paths for JBLM.

The above comment is accurate, plenty of local training helicopter flights will be fully or partly dark (lights and/or transponders off), looking at my receiver's raw output stream.

[miahi]

ADSB is not mandatory in the US below FL100 or FL180 (10000/18000 feet), that covers most helicopter flights.

It depends also on the website you are using to track. I have an ADSB receiver that publishes to multiple tracking websites (the same data, unfiltered), and not all of them publish all the data. Flightradar24 doesn't show most of the military aircraft - I can see them on my local tracking interface but they are not shown on their website.