A red button ?
Look at the Blizzard code for one of the frames that has it, I suspect it is one of the UI or options buttons.
Blizzard code is here if you weren't aware:
https://www.townlong-yak.com/framexml/live
One example to look at is : UIPanelButtonNoTooltipTemplate
xml code at :
https://www.townlong-yak.com/framexm...plates.xml#271
If not a red button maybe a screenshot or another permanent button that doesn't rely on a summons to see what you are talking about
The PetBattleQueueReady popup window ( code here:
https://www.townlong-yak.com/framexm...pupSpecial.xml ) has the following button used.
UIPanelButtonTemplate
https://www.townlong-yak.com/framexm...plates.xml#312