Hi Randy - I was able to duplicate what you are describing. I was logged in to Collector, then went in airplane mode, closed Collector and tried to reopen and was unable to do so. If I never closed the application it was still available to me. I also downloaded a map to my device and ran through the previous workflow. This time the application opened because there were maps/data downloaded to the device, and I was never signed out of Collector.
So the two things that made the app still functional in offline mode were making sure you never sign out of Collector (you can close the app) and making sure that there is data is downloaded and visible under the "On Devise" tab.
-Adam Z