[Rebelbase]

Cheap one

[Lurkinanloomin]

It probably is not set up to measure for radiation leakage which is measured in milliwatt/cm2.
It is probably just reacting to the electromagnetic field being generated by the rather sizable transformer used to power the magnetron (the part that emits the microwave energy).
Unless the door is damaged or the frame is bent, it is likely not leaking.