2007 Prius Gen 2 loosing coolant

If you open the radiator cap, not the reservoir, you may find that it's not full of coolant. While you're checking fluids, you should check your oil level too

 

So while you're driving, the triangle pops on for 2 seconds and turns off?

 

Rare to hear repetitive overheats as usually one good one will sometimes damage the engine
By warping th head and blown head gasket.
The fact that you lost a lot of coolant tells me that initial overheat was a bad one.

You never mentioned what code is the triangle throwing? And just because the dealer did all that work is no guarantee the air is out. Seen many dealers incapable of doing that right on this site.

Turn the heat on and drive for a while and see if you hear water gurgling noises under the dash.
Air trapped in system is main reason for over heat.

Buy an Xgauge and you will be able to monitor engine temp as you drive around. Can’t imagine chasing this issue without one.