2007 prius coolant has disappeared

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The plastic coolant overflow tank isn't a good indicator, please use the actual radiator for gauging the coolant level.

Did you notice any puddles of pink coolant (the color of the recommended Toyota SLLC fluid) under the vehicle after parking?

Given the mileage, when was the last time the ICE water pump was replaced?

 

+1 the reservoir is not a good indicator of coolant level. You should check the radiator and only be alarmed if that level is low as well

 

And check the coolant level in the What IS That thing...(inverter tank).

 

Agreed, but note that the cap is on a weird stalk that comes off the side of the radiator. Support that stalk with one hand while unscrewing the cap with another. Pretty sure a big guy could snap that thing off by pushing down too hard or using too much torque if the cap was stuck. Take off the black plastic front shield (the one over the radiator) first, for better access. Also, no tools are needed to remove the pins that hold that in place, just reach underneath with a finger and push up gently on the center pins.