Set during the three-episode arc, though I can't remember which one (probably the first, though). It won't really make sense unless you've seen this particular scene.
Passing
She's really just passing by when she spots him in the elevator, and can't help but get on with him.
But she's not going to say anything, because she knows why. It's painfully obvious why, especially since he'd been sleeping in his office again.
So Derek and Mark and Preston pass by, and she prays that they won't say anything.
But Murphy's Law triumphs again, and they do.
"He dyed it," Mark says bluntly, in a very Mark kind of way.
And because he's Mark, she lets him pass.
She lets Preston pass, too.
And hits Derek.
Reference: Murphy's Law: Everything in a given situation can and will go wrong. Words to live by.