Where does this come from?

Anyone want to try this:

Press and hold the right Alt key (keep holding it),
tap the left Alt key momentarily,
then tap the Tab key and tap Tab again to scroll through the XP style Tabs Menu,

... do you see a tab called Search Pane and does it bring up the 'Charms' Search Bar?

Been trying to figure out where this comes from!

Easter Egg ???

Oooo nice.

More than likely it's just a fallback. The problem here is that you're "locking" the menu mode on in a roundabout way, and the DWM rendering is unable to render the new alt-tab functionality in that mode, so Windows falls back to non-DWM alt-tab functionality. You would get the same if you disabled theming.

It's not an easter egg, it's more of a side-effect of the modal locking done by the menu system, combined with DWM rendering. In effect, it's a "hole" in the DWM renderer.