Details first, '14 991S and I looked at the search function without finding the information needed.

Last night on the way home, dash warning light for low tire pressure. Said "top up tire pressure" and indicated left rear tire "-4".
Looked at the TPS monitor and it showed all the tires at the normal setting, 39 R and 32 F. Tires are all season Toyo. Not the best but will get switched out in the spring.
Stopped at my local station and pulled out the electric tire pump (mine, not the factory one) and added air. It reflected the tire pressure on that wheel was 39. Added a little bringing it to 41, got in the car and turned it on.
Warning still there but now says "-3". Any suggestions as to what is going on?

 

The TPMS is reacting to a change from what it was set at.
Reset your TPMS system to accept your currect tire pressure as good.
Likely, the tires were set with one tire at 43 or so and with the colder temps has dropped some to set the alarm.
You could also drop them to the comfort setting of 32 and set the TPMS to be happy there.