According to the Operating Instructions:

The DC-DC converter installed in the microphone supplies....the audio amplifier and not only the microphone capsule. Since the converter compensates for variations of of the supply voltage it tries to do this also when the AC main is switched off.

Therefore the internal supply voltage...of the microphone is maintained for aproximately 2 seconds before it colapses with an audible "blubb" followed by a short noise.

Noises comparable to this can be recognized also when switching the supply on and it takes some seconds untill the microphone is ready to operate.