Unexpected voltage measured - explanation?

The goal is to avoid mold. It does not arise in direct dependence on the compressor, but on the humidity. Therefore, I consider it pragmatic to use this as a trigger signal. Tapping power somewhere on a device without knowing the circuit and the necessary device parameters is an unnecessary risk. I don't believe that an additional load or high-frequency power supply interference should pose a problem, but as is well known, the devil is in the details, and a clean solution is something else. Additionally, I think that intervention on the compressor voids the warranty. Also not ideal. Of course, your suggestion is feasible and I do recognize a certain "African" pragmatism (I know it very well from my family and sometimes appreciate it too). A simple plug-in hygrostat, for example from Trotec, will cost you €20 and works perfectly – you don't need to play around with a Raspberry Pi and sensors – unless you enjoy that anyway.

Thank you for the contribution. The tip about the Trotec socket hygrostat is a good one; I hadn't really considered such a thing before, nor would I have thought you could get such a device for an affordable price. However, its use would create an additional requirement: a socket in the area most at risk of moisture. Or an extension from the nearest one (above the refrigerator) down to the base. None of that is very aesthetically pleasing. I'll have to think about it. (oder: auch das am Kühlschrank hinten abgreifen :D:p ;) )

Extension cable up to the vicinity of the mold spot or moisture accumulation. With the setting, you can determine a threshold value at the location of the hygrostat, which is related to the value at the mold location - so it does not have to be squeezed into the narrow gap.