There are limits to where ADSB is required iirc. Near certain classes of airspace, above x altitude, etc.
As with any RF system above 30 MHz (the cutoff frequency of the ionosphere), ignoring reflections/multipath, a fairly unobstructed path from transmitter to receiver is needed. I have an ADS-B receiver with an antenna hanging over the edge of a desk, near the floor. It receives enough ADS-B to keep my free business-class FlightAware and FlightRadar24 accounts active, but not much more. Planes disappear from its coverage all the time.
So, I wouldn't impute too much affirmative information based only on negative information.
- H/T Moe on da boarf