On the back up bid... they could call days or weeks later if the previous deal falls apart. I would not hold my breath past the first 48 hrs but never know.
Hope that helps... buying a HUD can be a great deal but also can be more frustrating and have many different issues to deal with over a "regular" home purchase... I hope you are working with an agent that is experienced in helping home buyers purchase HUD homes?
Really it does not matter how they know, just the penalty.