I've never seen or heard of that happening before. My best guess here is operator error.
It sounds like the Project Panel is drilled down into the bin you were last working on. If that's the case, the Project panel's tab will read "Bin: <Bin name>" rather than "Project: <project name>".
One way out of this is click the little folder icon with the up arrow in the upper left corner of the panel.
Mark, Many thanks. Drilled down is a good word. I scoured that panel for icons that could help me, but missed that one.