“What are you doing over here?”
A simple statement, but potentially detrimental to you (and your career).
Here’s the context:
Unless you’re in an open-plan office, you’re most likely in Cubicle City. People tend to get insular about their section of the warren.
It makes sense; you’re there 8+ hours a day and this is your space within with your immediate department, even if you don’t particularly like it.
So when a department outsider enters your space…
SURPRISE!
Interloper!
Infiltrator!

Especially if you’re outside. This happens all the time.
If you find yourself feeling any of the above adjectives, or your own particular favorite, your role is to HIDE THOSE FEELINGS.
Chances are, that someone is in your area for a work-related reason.
All this should be obvious, shouldn’t it? Yet addressing someone with “What are you doing here?” happens a lot.
And the recipients remember their irritation when this happens, a lot. Even if it’s a known Office Wanderer or a self-identifying “Haven’t been over here in awhile” chatty type.
So while it’s certainly not the worst thing you could do or say at work, try not to say this one.