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.
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.
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.