Seems like Ian and Peter beat me to a response
So to back up a few of the statements.
Mode C/S transmissions are not automatic, they are only transmitted upon request, this means either from interrogation by Ground Radar, or a TCAS system. its unlikely to see an interrogation on the ground, the other aircraft may have been interrogated by a TCAS, or hit the 'ident' button, which I think forces a response.
Also regarding Mode C/S, remember that all reception does not necessarily generate a warning.
If you want to see what PilotAware is receiving in terms of Mode C/S, take a look at the traffic page.
These signatures are only forwarded to the navigation device, based upon certain criteria
- relative altitude
- sensitivity
If you are not seeing Mode C/S listed in the traffic page - there may well be a fault, but I think this is unlikely
The acid test is to take a look at the traffic page, and determine whether it is showing reasonable information
thx
Lee