<RSBM>
The only way I can make a random scenario fit is if William was actually out of FM's sight for longer than she has said, whether because she simply doesn't have a clear idea of time or because she was worried about the consequences of saying he was gone for, say, 20 minutes. This gives him time to get much further away, perhaps where it is obvious he is not near his family, and somewhere less visible. It also gives a bit more time for someone to make the decision to take him.
I can understand the initial commentary that it was only 5 minutes, but I have never understood zealously sticking to it, whatever the original reason. If it was me, I'd be saying, "Look it could have been longer" even if I didn't think it was longer just to broaden the options of what may have happened, and because I would never trust that I could be so accurate about the time under those circumstances. To me the difference between 5 and even 15 minutes is significant here. That is not, BTW, a suggestion of guilt about anything.