Wait, TRE... was the March Ring in her inventory? If so you should have been able to steal it back. You can steal from Flayn there, she had a Magic Staff (and an Elixir, I think) when I played and I stole it because on my playthrough Byleth is a kleptomaniac. Can't grab Seteth's relic though.
No. Her inventory does not include items she took with her when she left. So the March Ring was just gone. At the time I was just still hoping there was still a way to recruit her back and that she would still have it. Getting Flayn back would have been nice too. >_> ---
Link isn't the only one who kicks ass. Congrats to Advokaiser for winning the CBX Guru Contest!