That result appears to confirm that you don't have the type of head gasket failure that you also don't have symptoms for. Back to a simple but long-unresolved coolant leak. And honestly with this many overheats over this much time, I wouldn't be shocked if the head were warped a little, which could have exacerbated a small old leak or even opened a new leak altogether. Find the leak.
These cars don't "use" coolant with no leak or without a head gasket fail. It should go for ten years without losing coolant. Low coolant obviously causes overheating. You have a very real problem. Most likely he was right. It is a design flaw with these cars. Head gaskets and water pumps are common issues. Water pumps can gradually go bad and not leak. They may fail to pump 100% of their expected volume. Normally an overheat from a water pump will cause the coolant reservoir cap to release coolant. Lately it is head gaskets more than anything. The guy before you could have clogged it up with sealant when the hg failed. Advice that suggests combustion gas detectors or water in the oil are definitive of a hg are wrong on this engine. A borescope pressure test is definitive. $700 for radiator fans and sensors that did nothing tells me one thing. You need a better mechanic who knows Prius issues. Find a competent Prius mechanic who has seen these problems. By now you likely need a replacement engine.
I'm really surprised something so catastrophic isn't met with any vigor on the dashboard, besides an ambiguous red light that pops on for seconds and goes off and appears again in 2 months. Surprised I even noticed it given how briefly it appears. Before the mechanic suggestions, I mentioned the quick red light pop on during oil changes this summer to find out what it could be, and they just thought it to be coolant fluctuating because the light only appears on a hill incline. It's not that I'm ignoring a steady red light. Starting to get toxic here like most forums do, but thanks for all the insight everyone. I'm a driver, no mechanic. I'm all about preventative maintenance and haven't been driving the Prius since this post. Wish I had a mechanic's understanding to perform some of the more advanced of what's been suggested. I can tell this comes second nature to alot of you.
