As I said in my previous post, if he gets the BM2 battery monitor, he will find out of it is working or not, as the solar panel will start charging the 12-V battery.
..... And - only if it's a cloudless day at that minute of the day, AND not extremely hot as panels easily lose 15%-20% of their efficiency. AND - That highest Angle of the Sun At best Is Less than an hour before & less than an hour afterwards. Azimuth before & after perfectconditions being lower than perfectly ideal. .
I doubt it. I doubt the solar panel turns on the main DC–DC converter to aggressively charge the 12-V battery at around 14.1 V. In the Gen 4 Prius Prime PHEV, there is an auxiliary DC–DC converter in the AC charger that keeps the 12-V battery at float at around 13.0 V. This doesn’t really charge the 12-V battery but prevents it from draining during AC charging. It is turned off when the traction-battery SOC reaches 100%. The main DC–DC converter is not turned on during AC charging. I am guessing something similar is done by the solar-panel computer and it has an auxiliary DC–DC converter in it to provide a small power to the 12-V system. Note that in Gen 5, the main DC–DC converter is turned on during AC charging. Again, all these answers can be provided by getting a BM2 battery monitor.
As some others have said, just take it in. They'll plug in their computer and see if it's thrown any codes. If it hasn't, there are troubleshooting steps in the repair manual that they will follow to track down any problems. Just note that it won't be particularly quick. Unless it's a huge dealership, it's likely they'll only have a couple/few techs that are qualified to do the work(probably only Certified Tech, Master Certified Tech, or Master Diagnostic Tech). Depending how busy they are, they might not have a spot for you for awhile. Then, they'll only be able to do the testing between 11am and 2pm on sunny or mostly sunny days(that's part of the repair manual procedure). So it might take them a few days to track down the problem if it doesn't show itself right away.
i also had the 2010 version and even with snow on the roof here in Canada, i'd still occasionally hear a momentary sound from my fan.