Austin and Ally don' question Trish and Dez arriving back from the convention late in Dez' dream, when it's starting to get dark, and when they've taken off their costume.
Upon waking Dez up from his dream, Austin tells him that the Convention had already been over, yet in the dream, it had been going on overnight and into the next day.
No explanation was given by Zip and Zilch as to what people would've done at the convention upon finding a real Zalien Egg, which is distinguishable for being unbreakable.