Failsafe mode!

Buying one of these will allow you to use phone apps, like Dr Prius app to read error codes and clear warning lights: https://www.ebay.com/itm/265484403768 Alternatively you can disconnect the 12 battery for a few minutes to wipe car memory of error codes and settings and it will do a fresh re-boot of all the systems until it detects the problem again.

 

The most important thing to do is to find out what the trouble codes are. If the mechanic plugged in a scan tool and it did not retrieve any trouble codes, then they didn't use a suitably capable scan tool, and you could either acquire one yourself (never a bad idea for a Prius owner), or borrow one from somebody (or just borrow somebody who has one), or go to a dealer, where they will have Techstream to read all the trouble codes.

Ideally, finding out the trouble codes should precede clearing them. By clearing them, you might be able to get the car to forget it has a problem, and be able to drive it around some more that way before it notices again, and until it notices again, you kind of forfeit your chance to find out what the problem is.