Red triangle, temperature warning

Reading the error codes and subcodes will give us more specifics. Do you have a friend who has Toyota Techstream software on their laptop? Or maybe someone with and OBD2 reader so you can use Dr. Prius app on your phone? Here's buyers guide for OBD2 reader: Hybrid battery diagnostic and repair tool for Toyota and Lexus

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Higher speeds usually require more engine power, producing more heat. It's possible there isn't anything with "issues at higher speeds", but just that somehow the cooling system's overall heat-removing capacity is reduced, and you hit that limit at higher power levels.

 

Fluid being visible at the passenger side front is often indicative of the water pump seals failing. The water pump is located on the passenger side front bottom area of the engine. Take a flashlight and look at it. You specifically want to look at the back of the pulley, because coolant will collect there when the seal is failing. It should be a pink color. Slow weepage usually results in a crusty pink layer in the backside of the pulley. A more significant leak leaves it wet. If it's dry and crusty, it's not an immediate crisis. If it's wet, you better get on it.

Most often, the people compartment heat being intermittent is a pretty reliable sign that engine coolant is getting low, regardless of what's in the overflow tank.

 

Hope your not using the over flow tank as a gauge of coolant in the rad as that does not work well.
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Take the plastic cover that's on the top of the rad off its six screws under there is the rad cap. I bet the rad is pretty empty hence the thermometer on the dash.

 

This could be a sign of:
Water pump failure.
Plugged radiator core.....or plugged external fins on radiator.

And I hate to mention: Leaking head gasket. Check the oil for signs of water.