Tooth Fairy Mystery

M. lost his tooth at school.  We forgot to put it under his pillow so we did it the next night.  I remember thinking "oh I am too tired.  I better tell D. to do the tooth fairy duties before he comes to bed."  And that was the last I thought or did anything about it until the next morning.  M. was already  awake but hadn't looked under his pillow yet.  I couldn't think of a way to get in there and make the exchange without him seeing me.  He knocked on the bathroom door to tell me the Tooth Fairy left him a gold coin, but that the tooth was still there.  I explained that TF must have been in a hurry since he'd said 4 kids in his class lost their teeth over 2 days.  She'd just come back for it later (but he wouldn't get any more $).  Later I thanked D. for remembering. He said he didn't do it.  I told him to stop kidding.  He was insistent that he did not put the coin under the pillow.  I know I didn't.  So who did?