The poster on the other thread said he was experiencing a single line decode on FT8 with his callsign and the other station's callsign and RR73 or 73, just out of the blue. 

Over the past year or so, I've had many of the RR73 or 73 single line decodes.  None had the "ap" signatures.  Looking back, I see the other station was actively calling and working other stations normally.  

My take: since they were all "foreign" stations, I suspect that they were hoping for a confirmation from me for my state to put another "pin" in their WAS attempt.  Of course, I just ignored them and moved on.  No logging happened. 

Another thing that does not get logged, is when a station answers my CQ with RR73 or 73.  This is also typically a "foreign" station.  What are these folks thinking? 

This all reminds me of all the messages and email I received while I was uploading to eQSL, from those saying I worked them, but why were they not in my eQSL log.  The reason being is that I never worked them to begin with.  I expect this was all the same as above.  So 2 years ago I stopped uploading to eQSL and the messages and email stopped almost immediately.