Nancy Grace tried really hard to get an answer on this. It seems unlikely she placed anything in the bag after going through the trouble of undressing her. I believe the location, fact that there are no other babies the right age and Caylee's lovely brown hair where the identifying factors.
Then LE went back to the house to find the tape and possibly the bags. IIRC from the Greta interview, GA had those bags in his shed, unlike the white one with the 'trash' in her car.
I could be wrong, it happened once in 2002! No, no, I thought I was wrong once in 2002, but I was mistaken.