This is an action birds do to try and scare us/predators away. Also it could be a way of them cooling themselves down. If there was a problem with respiratory, then there would be a sort of clicking noise when they are breathing, or more tail bobbing. Try to keep an eye on them while hidden from them to study this behavior.

My zebra finches sometimes hold open their mouths for a few seconds. They are well in themselves and are eating/drinking normally. I just wondered is this a normal behaviour for them to hold open their beaks and look around?