Was coolant boiling in overflow tank when you saw it?
The wife topping off the coolant is suspicious on its own.
I mean why?
Has there been a slow leak that would cause her to look and top off? Again, why?
If you cannot recall when the radiator cap was replaced, then replace it with a genuine Toyota cap.
Luck and enjoy the Runner.
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2001 Limited 4WD - 346+K - SunfireRed\Thunder Cloud; - 265/75/16 Michelin A/T2s - Fat Pat's 1.5" BL - StopTech ANGLED rotors - In series 699 trans cooler, New Yota1 transmission, All new OEM suspension front to rear.