Having read this through I'm left thinking that everyone has become fixated on the object (the phone) and what was done with it.
Is it not entirely possible that the "security issue" was not the phone itself being a risk as such but that it indicated the a/c had not been properly secured or checked.
In years gone by I've certainly accepted "lost & found" items dropped out the window to me after doors closed and saw no problem with that.
In this particular situation it may be that someone decided that it suggested the a/c may not have been either secured overnight or an acceptable cabin check carried out prior to boarding.????