Just a shot in the dark here, because the same thing happened to me once and it took a while to track it down.
Check for a VERY slow leak of coolant from the cooling system.
I needed a cooling system pressure tester and patience to find mine, as the engine would evidently only ordinarily lose the coolant above a certain temperature and PRESSURE, (which was not being reached unless the car was being driven). Even with the pressure tester, the radiator cap was fine, and the system initially looked to be leak free, but after leaving the tester pressurized to approximately the upper limit of the standard radiator cap, and taking a short break, the leak began to be noticible. This time lag may well have been because the coolant was not hot at the time the testing was being done and differential thermal expansion was not taking place . (In my trucks' case, it turned out to be just a very slightly too loose hose clamp on a radiator hose which had been replaced several months earlier.)
If there is a leakage path for coolant, the same passage can admit air once the engine is shut down and cools, as then the coolant volume decreases. (This is when the coolant is normally sucked back into the engine from the expansion/overflow tank.)
This results in some air being trapped in the cooling sysyem. On many engines, when air is trapped in the cooling system, the internal metal of the engine adjacent to the area where the air is, can get much hotter than if it was in continuous, direct contact with the coolant, so, when coolant DOES contact that area, that coolant can get superheated and turn to steam, which will raise the temperature (and pressure) of the rest of the coolant. If the leak is severe enough to allow a lot of air to be trapped, the resultant pressure buildup can blow off a hose or a fitting, or even burst a radiator, whichever is the weakest link in the system.