Did you have a companion at the time? Their ultimate icon appears between the quickslot and the player's active skills while they are summoned and goes away when not summoned. Y is the default keybind for controlling combat pets and companions. Ergo, logic dictates that it may have been for the companion's ultimate.
PC-NA / PC-EU
ID @IsharaMeradin
Characters NA
Verin Jenet Eshava - Dark Elf Warden (main)
Nerissa Valin - Imperial Necromancer (secondary)
Lugsa-Lota-Stuph - Argonian Sorcerer
Leanne Martin - Breton Templar
Latash Gra-Ushaba - Orc Dragonknight
Ishara Merádin - Redguard Nightblade
Arylina Loreal - High Elf Sorcerer
Sasha al'Therin - Nord Necromancer
Paula Roseróbloom - Wood Elf Warden
Ja'Linga - Khajiit Arcanist
I believe that is for a companion ultimate, but not quite sure. At least when I have a companion high enough to have an ultimate, that is where it goes, and Y is the default for that.