2002 Subaru Outback Temperature

I have a 2002 Subaru Outback 2.5, and I recently had the head gaskets replaced by a professional mechanic (a few weeks ago). It had an external coolant leak, but it had not yet overheated, so my mechanic replaced the head gaskets along with the water pump, timing belt, thermostat, hoses, etc while he had it apart.

It has been running great so far, but recently I noticed the temperature gauge rising a little above normal. My temp gauge is split into equal fourths - 1st dash is "C" cold, 4th is "H" hot (overheating), and there are two dashes in between.

Usually, it rests dead center between the two inner dashes, but I have noticed it rising to the 3rd dash and staying for a while. This happens when I am moving along at speed in 20 degree (Fahrenheit) weather. I find turning the heat up higher brings it down.

It hasn't gotten past the 3rd dash, or into the red zone, but it does seem higher than normal. Should I be worried at all?

The radiator is full and the overflow good too, as it's been since I got the car back, and there are no leaks apparent.

I had my mechanic take a look at it today, and he said he couldn't find anything, but replaced the thermostat as a matter of course. Later today, I took a longer drive to test the car out (sustained 50-60mph in cold weather, heat on low). The temperature went and stayed at the middle for a while, then slowly went to the 3rd mark, then slowly went back down, and stayed. What could be causing this variance? Is my gauge maybe not reading correctly or the temperature sensor? When idling, both fans turn on, and keep temperature where it is, but don't necessarily lower it. This seems to just be an odd intermittent problem. Thanks!