We have had issues where Collector signs user out when they are in a disconnected area. Is there a bug out here where this is a known issue. Either way the users a left hanging until they are able to get to a spot where they are connected again to sign in.
I have also encountered that problem and asked Esri when Collector needs a login, but I haven´t got any answer.
A workaround for iOS when asked for a login in a disconnected area:
1. Enable Flightmode - Close Collector (Swipe it off) - Start Collector again.
It have solved the problem for my users, but I would prefer a better solution.