Hi! My 2007 Prius started showing the red triangle after sitting in the garage for three months , with only being started and idled for a few minutes every weekend. It drives fine. The dealership completed a 10K servicing and found nothing wrong during its checkpoint inspection. I’m still waiting to be told the fault codes but they recommend replacement of the hybrid battery for about $3.5 K. I never had an issue with this car though it has a couple of dents from stupid driving mistakes. All recalls were taken care of and all servicing has been done with the dealership. I took it for smog testing to be able to sell it and midway the red triangle and check engine lights went away. Smog test is complete. What happened here? Will it keep for a couple of years without a new battery? Should I sell it for nothing and be done with it. I am in California.

There is NO good reason to do that......and several good reasons to NOT do it.

The best one probably is: It takes more than a few minutes to put back the charge into the 12 V battery that is lost from just sitting and starting. The starting also draws from the HV battery and it won't fully recharge either.
Then there is the condensation in the exhaust and crankcase that forms from short runs like that.

All you really need to do it connect a battery tender to the 12 V.......or actually drive it 10 miles or so.

I suggest that you get the 12 V tested.
And that price for a battery is a bit high. If you end up needing one, shop around.
But avoid places that "rebuild" batteries by replacing only the failed cells.

is Getting the 12V battery tested different or in addition to getting the diagnostic work up? I paid for the work up separately from the service but there are no codes listed on my paperwork. The service person mentioned code P0A83 but says that he will provide the results on Monday.
I am trying to locate a hybrid mechanic.

Probably. Ask them if they tested the 12 V....and how they did that.

Amazingly, it seems that a lot of what are supposed to be competent shops don't know HOW to test a battery......and know one is bad only if it is totally DEAD.