They weren't planning on it, but the information was sent nonetheless. P(Someone is going to go back and stop them from going back|They came back) < P(Someone is going to go back and stop them from going back|They did not came back)

That presupposes that P(Bob came back) is not affected by your decision to send the information further on. I'm postulating that IF you would have sent the information further back, THEN P(Bob came back) = 0. Of course, it might not actually work that way, but if my supposition is correct, then Bob not coming back tells you nothing. The event only carries information if you aren't going to make use of that information.