Yeah, filtering is one possibility. They could also be switching the transponder to Mode 3 (equivalent to civilian Mode C) which is not ADS-B compatible but wo

Yeah, filtering is one possibility. They could also be switching the transponder to Mode 3 (equivalent to civilian Mode C) which is not ADS-B compatible but would still show up on normal normal civil radar scopes. Or they can use some military only modes. Some of those could in theory be shown on civil scopes, but I'm not sure they actually are, while others like mode 4 or 5 cannot. But they could show up on the UK's military scopes, given nato intercompatibility. Another theoretical option would be for the air force planes to be using mode-S, but having disabled mode-S extended squitter (which means the planes don't periodically broadcast their location, but simply only reply when interrogated by radar). I would expect ADS-B receiver to pick up on these periodic interrogations though. I'm not sure if interrogation responses are directional, but even if so, some receivers would probably be in the general direction of the radar site. Its not really clear which. Outside of wartime, normal US Air Force policy is to fully integrate into the civil ATC system, except in special military use/training areas or certain special missions. This integration just makes things safer for everybody.