If you work in a small firm, where everyone works together, then the simple answer is DON'T EVEN THINK ABOUT IT! Everyone will know about you, no matter what you do/say. As Archer1000 said, why risk your job over some good looking girl, who you don't know, when there are probably a dozen more outside, who you can approach at lunchtime!? Thats the beauty of daygame, approaching girls anytime, anyplace.
If you work for a big multi national company, with several office buildings/departments, then the risk is lower, so you could follow Rob's advice. Just be warned that even then, if someone who you know/knows her, spots you both in Starbucks, you still potentially face having your personal life emailed around the firm! If you know the girl and like her and your looking for a long term girl friend then I could understand you doing it, but if it's just to have a fun, casual fling then why risk your career prospects!?