How do you know it was a 397 ghost? You can't check the light until you actually visit the postmaster. Is it possible that it went to your postmaster, you saw it was 397, and then you left it there to do more grinding? If that the likely case of what actually happened, items rotate through the postmaster, with newer items that you leave behind (including regular ol blue engrams) replacing older items. Items that get left at the postmaster and are cycled through are sadly gone forever. Chalk it up to a lesson learned and next time either take the 397 item right away (why you didn't in the first place is beyond me, honestly), or move it to your vault so it doesn't cycle through.