it's most likely a <div> with initial display value of none (display:none) that switches to (display:block) when the plus sign is moused over.
there are other js that go with this, possibly
- a script that places the "+" sign in a fixed place
- one that places the pop box relatively to the mouse position
- one that defines the clickable/acrtive area
anyway, at its core it's probably something like this:
conversely, you'd need something on mouseout that does the opposite...