You are spreading false information. A microwave oven does not "leak" or generate the kind of dangerous alpha particles, beta particles, and gamma rays (ionizing radiation) detected by a Geiger counter.
Is their a decently affordable type of tool for measuring or capturing em radiation in the WiFi frequency range? I suppose I could glean a lot from running a logger on an ESP32 and walking around, but I have to imagine EE folks and scientists have better tools for the job.
... along with a high dB external attenuator because it is designed for low power. According to the docs a 60dB external allows for 1 milliWatt to 2 Watt measureable input. I think the default power for 2.4ghz Wifi is 100mW.
Microwave ovens operate at 2,450 MHz.