Inverter coolant low on both my Prius's

Could just be evaporation. For comparison, with our 3rd gen I've had to do some top up, over the years. The timeline:

2010_11 - purchased car
2015_05 - 52k kms - both reservoirs at low line, topped up
2018_10 - 78k kms - engine reservoir only at low line, topped up (inverter reservoir never budged)
2020_10 - 88k kms - both reservoirs still at full line, changed coolant in both

 

When you say "Low" what exactly do you mean? Is the level on the low mark or well below the low mark?

If it's simply on the low mark, I would not worry. Anywhere between low and high is acceptable.

 

surprise, surprise - a dealership not doing their job. Not really. A busy dealership is a production line and mechanics that can push more cars out the door are rewarded. It's no surprise that some unscrupulous mechanics will simply check things off a list and not even look at it. That's probably what happened. There's almost zero profit margin in standard OM maintenance per OEM manual - they make up for that in additional and/or un-needed services. ie.. your expensive diagnostic fees. Hopefully they don't replace something that doesn't need replacing, just to get a few extra bucks from ya'...
Dealership will complement you on your attentiveness, while they're sticking a crowbar into your wallet. Thank You for catching our mistake and here's your bill for $1K, services rendered. Win-Win!!! well more so for dealership.

1. never assume
2. could've saved a lot of money by just topping it off and checking it, for the next few weeks. If it continues to go down, take it in. If it stays put, you just saved yourself a bunch of diagnostic money and replacement parts that wasn't needed.