They're really good at hiding joeys at times, over a year ago when Oti went OOP I thought during 3 weeks that he was alone, and then one evening his brother Teeboo mysteriously appeared in the nest alongside him, a little smaller but not by much.
I never found out where they hid him for so long, he was too big to just have been hidden in Frejya's pouch and I was manipulating the joey every evenings so I'm quite sure he was the only one in the nest... The only explanation I could come up with is that perhaps their sisters were babysitting one of the 2 in the other nestbox each time.