In my years of recording sound effects and dialogue for film and commercial production I have hidden microphpones, mostly lav microphones, in all kinds of places (as I'm sure most sound recordists have).
I've planted microphones in a tree branch in a West Virginia field, a toilet bowl to record flushing sounds (yes, I used protection...), heck I've even planted a microphone in a plant (a Poinsettia, to be exact). Last week, however, while on a shoot in Ireland, we traveled to the Annaghmare, a neolithic tomb, to film the Armagh Rhymers, where I planted lavs in the strangest place yet.
I carefully taped up the lav microphone and transmitter inside their delicate, woven horse and bull masks. I had to race the quickly setting sun, but I prevailed! The resulting sound was clothing rustle-free (an annoyance and challenge to any sound recordist hiding microphones under talents' clothes), and with the additional boom for a more distant perspective, I captured enough tracks to create a nice mix in post. We recorded the Rhymers' voices, reciting poetry by Joseph Campbell, the bodhrán (listen to a bodhrán sample from the Foundation Library on download.prosoundeffects.com), and the bones.
So tell me, where is the strangest place you've ever planted a microphone?