I've noticed on recent trips to TNK the 'R' flashing in the Garmin transponder display as if it's still identified on radar. On the NOTAMs TNK is listed as a recent site addition for the system...
In line with the reply from SM4 Pirate, your transponder can be interogated by any SSR facility. This includes military equipment either ariborne or on the ground.
I'm not at work, but is TNK Tennant Creek? If so, you're in an area that is full of both airborne and ground military radar equipment that could very well be interogating your transponder.