Sometimes I get a stray frame like this:

The name of the frame is WQTItemTooltipScan and it is anchored to the mouse. It seems to appear randomly, but when it does, it only appears right after login; a reload is required to get rid of it.

Grepping through my AddOns folder brought me to the only possible non-Blizz origin of this frame, in WQAchievements/WQAchievements.lua, line 882:

ItemTooltipScan = CreateFrame("GameTooltip", "WQTItemTooltipScan", UIParent, "InternalEmbeddedItemTooltipTemplate")
ItemTooltipScan.texts = {
	_G["WQTItemTooltipScanTooltipTextLeft1"],
	_G["WQTItemTooltipScanTooltipTextLeft2"],
	_G["WQTItemTooltipScanTooltipTextLeft3"],
	_G["WQTItemTooltipScanTooltipTextLeft4"]
}
ItemTooltipScan.patern = ITEM_LEVEL:gsub("%%d", "(%%d+)") --from LibItemUpgradeInfo-1.0

I couldn’t find any references to ItemTooltipScan in the rest of the addon, and also not in any other addon I have in my current AddOns folder or in my Archived AddOns folder. So, I’m wondering what the purpose of this is?

If the frame is needed for some interaction with an unknown addon, then I suggest creating it only when that other addon is loaded.

I’m currently running WQA with the entire block (lines 882–889) commented out, and everything seems to be working fine, so far.

PS:

It occurred to me now that “WQT” could mean ‘World Quest Tracker’ (the Terciob addon). I downloaded the addon, and in fact it it also creates a WQTItemTooltipScan frame in WorldQuestTracker_MapAPI.lua, 478:

local ItemTooltipScan = CreateFrame ("GameTooltip", "WQTItemTooltipScan", UIParent, "InternalEmbeddedItemTooltipTemplate")

I’m not using World Quest Tracker, nor do I have it in my AddOns folder.

At first glance it looks like you're right and it's useless now.
I suspect this is a relic of an ancient time when it was necessary to scan tooltips to get certain information.

useless now

Above all, it produces a stray frame (see screenshot in OP).

Since I've posted, I'm running WQA with the offending ItemTooltipScan lines commented out, and no problems so far. And the stray frame didn't appear again of course.

But as said, I'm not using WQT, so I cannot tell if removing the lines cuts off some interactive functionality with WQT. But looking at what WQT does in regards to the WQTItemTooltipScan frame and ItemTooltipScan table (which is local in WQT) and what WQA does, the code doesn't make any sense – to me.