Just a side note- a few people are saying the park was SO FAR away and there is NO WAY a 3yo would have walked there. I just want to dial that back a little bit. According to reports, the park where Brendan was found is about 1/2 mile from his dad's apartment, and it is straight down the street. I have personal experience with a 3yo and 2yo wandering off, and being found 3/4 mile from where they started. That story, luckily, had a happy ending, as they were found unharmed. So it is not unheard of or even unlikely, IMO, that a 3yo could wander down the street to a park.
I realize it has been indicated that Brendan was "placed there" so I don't even know if the possibility/probability of him walking away is even a factor. But he could have gotten some distance from home on his own.