Well I don't know why they would not search the taxi. That does not make any sense. I bet they did.
Only if it was available at the time and he gave them permission. Also, they may not have wanted to spend the money it takes for a forensic analysis until further evidence was obtained. Like a video, for example.
Another thing - if the cab company has more than one sedan that looks the same, he could have given them the wrong one to search.