Recently, I had company for the week and had to go pick up my son from his day camp. On Friday, I left my 2 yr old with my guests and thus, went to get my son alone. I was shocked at the number of people whom I passed between my car and the classroom where I fetch my kid every afternoon who asked about my 2 yr old daughter. "Where's the baby?" "missing someone, aren't ya?" These are people whom I don't even know, just employees, other parents, folks associated with the science camp. I swear, even the homeless guy who sits on a bench outside the museum that was hosting the event, asked me if I had left something important at home. Since he was the first one who spoke to me about her, at first I didn't even realize what he meant. People noticed her absence and it really surprised me.
So, I agree that we haven't heard that the baby was with her in the school, BUT I think that people who saw her would have mentioned the lack of Baby K long ago, and LE would have asked about it (but very likely they asked about that anyhow) What could she have said besides the truth? And how could she have known that nobody was going to mention the lack of Baby K to have concealed the babysitter till now? She really couldn't have lied about it... "oh yeah, that's right... I had a babysitter in the car.. one who would have been able to verify that I came out of the building all alone" So, although it makes perfect sense that another person in the car=babysitter, I don't think it could work in this case unless LE has known all along and either known the identity or known that she was concealing the identity.