I don't know, one idea is that during the searches, one of the searchers or staff parked their vehicle near lamppost, entered 5A, at some stage needed bathroom, washed hands, dried on towel, then later walked back to their vehicle parked next to lamppost. That scent might be stronger on towel than child's?
I didn't see in the files any mention of the unknown searchers entering the apartment, let alone washing their hands in their bathroom. I thought if this had happened someone would know this. And do we know if this towel was in the bathroom at all?
Don't forget this same trail was picked on the 8th. This means 'the person' never walked on this route between the 4th and the 8th.
So we have someone who only did walk on this route either prior to 4th and never again or prior to 4th and never until after the 8th?
What I said before, from the phone records it would be easy to find this person, if the person exist.
So, for this theory of yours:
1. The person had to be inside the apartment and relaxed enough to be able to to go into the toilet there.
2. The person had to never walk on this route again, or between before 4th and 8th
3. Person had to have departed by a car.
IMO, there is a bigger mathematical chance this was Madeleine than the person from 1.2.3. but if the person existed it would be possible to find him-her.