This happened to me, it was with a coworker who I met while on a temporary contract.

We started flirting pretty soon after we met and she made very strong hints about how she was single e.g. "oh man my (female) roommate is annoying me by making noise in the kitchen late at night" or "what did you do this weekend, I was just at home feeling lonely".

I was really into this girl, not just for her looks, she was funny, cute, as smart or smarter than me, great education, the whole package and in my head I thought this could be "the real deal" if you know what I mean. Still, I didn't ask her out even though she was dropping hints because we worked together and I don't do that. Eventually she just started asking me out to stuff - "there's this great brunch place do you want to go this weekend" or "do you want to go to a karaoke bar with some of my friends", etc.

About 3 months into this the contract I'm on is winding down and I'm fully going to ask her out right when it ends. Then I find out through a coworker who saw us flirting that she has a boyfriend she hasn't mentioned this whole time...

When I bring it up she goes beet red and then mentions that yes, she is seeing someone and its a long relationship not a new one.

To answer your question, what I did was nothing, didn't ask her out at the end of the contract, just said "see you" and then she actually texted me for a booty call a few weeks after that and I was really tempted but said no.

What I kind of wish I had done:

Me: "Hey, I hope you know I'm into you, you are really special. Go out with me?" Her: "You know I have a boyfriend" Me: "Yea, I know. Um, break up with him."

Then if she said no I'd respect her wishes and tell her he's a lucky guy or if she somehow decided to do it I'd count my lucky stars.