I agree, if your CPU cooler fan is positioned a few mm from an external vent panel, then the idle temps shouldn't be much above what you would see in an open air scenario, maybe within 5-10C.
However if the gap is around 20mm or more then its a completely different story, you will start to get a large amount of heat re-circulation even with a vented panel, I could totally understand those temps
@Stan68 if that is what has been happening in your case.
I once tried to build an AMD A8-7600 system in the
Akasa Crypto X1 using an this
Akasa LP cooler, but the CPU and system temps were just ridiculous, even when idle it gradually warmed up and up until the fan was maxed out. I figured out that it was the distance to the vent above the cooler which was the main factor. The low profile cooler was just re-circulating its own heat rather than pulling cool air in through the vent.
If your case doesn't have a dedicated case fan then the CPU cooler
MUST also become the case fan as secondary function. Heat re-circulation is your enemy.