(edited by Belorn.2659)
Glory booster from living story
I was awarded one when i first began the newest LS. It can’t be used and requires the annoying dialogue in order to be destroyed.
What happened is that there is a maximum number of story/game mails your mailbox can hold (counted separately from player mails). When you exceed it, the game deletes the oldest one. If there are any items in it, they get deposited in your inventory.
Your oldest mail just happened to be the one from around a year ago when everyone got a glory booster. The same thing happened to me, except my oldest mail was the one from Scarlet with Scarlet’s Gift.
https://www.reddit.com/r/Guildwars2/comments/2c5zl9/i_just_received_a_glory_booster_after_using_a/