There is this girl in my work that I often flirt really heavily with, we go as far as to touch each other 'inappropriately' as others would say.

I would not have sex with this girl for a variety of reasons, looks is not one of them though, she is a HB 7.5/8.

Although I am certain she thinks I would have sex with her.

There is another boy in my work that she has a thing for, (they kissed on the staff night out) she try's to make him jealous by flirting with myself and I can tell she assumes I'm none the wiser.

The flirting isnt something I can just ignore, she is a manager in my work and she grabbed my hand and took me to do an assessment and decided to do it with me right next to where the other boy was, what a coincidence! She comes up behind me and feels my bum infront of the boy and says things like 'oh why must you tease me?' when I'm bent over

I was considering saying to her the other day: 'You know I do understand what you're up too'

I wouldn't directly disclose what it is I 'know' though, I would leave it with a hint of ambiguity incase she herself didn't even realize she was doing it, however unlikely.

What are your opinions on calling girls out on things like that?

I don't like being used by a woman attempting to further her relationship with another man.