Inverter Coolant Reservoir cap

First, welcome to PriusChat.

What you knocked off appears to be a vent port on the low pressure side of the inverter coolant surge tank. It doesn't look like a fitting for a hose as the hard plastic vent is split along the bottom and has a black rubber cap in place to perhaps keep any coolant escaping there from spraying on the engine. Not sure what the temperature operating range of this coolant loop is, or if it actually becomes pressurized during use. I would make a close examination of the white plastic around the busted vent to make sure there aren't any cracks extending into the cap area that would allow coolant seepage. A replacement would likely consist of the entire tank and cap assembly, and as with other parts rather $$$. I would watch it for a while and make sure there isn't any pink crust forming around the area indicative of a leak.

 

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