The most likely cause for a detector "over-heating" is caused by either direct exposure to sun and heat or confined storage in a heated environment.
Never leave a detector in a car trunk where it can get very hot in direct summer sun. Never leave one in one of those black plastic luggage totes on a vehicle's room, exposed to a lot of direct hot sunshine. Never leave them in a vehicle, such as on a seat, exposed to direct sunlight.
I never use a trunk or car-top carrier. I do, however, let my detectors travel with me almost all the time on the van seats. I have used precaution for several decades now by covering them, mainly the control housings, with a very white blanket. Nothing heavy, but like those white blankets the warm up at the hospitals to cover a nearly naked patient with. Being white they don't absorb the heat of the sun, and they are light weight to allow more free air movement.
Whenever I can I try to park so the areas is shaded. Also, I leave a couple of windows cracked open a little for ventilation. If your vehicle has rear windows that are very tinted, that can also help. I enjoy detecting and try to always be at the ready to get out and hunt. If my main use detectors are always with me, then I am always ready.
There are two times I remove them from a vehicle. In the highest heat of the summer when there will be a few days in a row when I don't want to be out in the heat. Also, during the coldest snap of winter, if we have a number of very cold, sub-freezing days or nights, when ... again ... I don't want to get out and experience the misery. I don't mind some of those days that drop down around the freezing mark but warm up into the 30's or 40's when I still might look for a warm sunny area to hunt. Just the extremes, either direction, for an extended period of time.
Monte