Well you have a choice; you can either let it carry on the way it is although it's plain to see you're unhappy about it. You could speak to your BF about it: see how he feels... maybe get him to have a word with her and let her down gently. Alternatively, you could speak to her and make your feelings known.
Bear this in mind, however:
1) As you said, she's on the rebound, so this could just be a phase that will stop over time.
2) Your BF may be offended or think that you're a bit of a 'bunny boiler' if you seem overly concerned about this (I assume) quite minor flirting.
3) She's a cop, but still human and so does have emotions which could be somewhat erratic. Tread carefully if you confront her.
Personally, I'd get your BF to have a quiet word with her at some stage.
A little more detail about your relationship with your BF would be useful: how long you've been going out, how long you've lived together, etc.
Good luck, and post up what you decide to do!
Edit: Didn't know you're a gay couple... you're sorted really aren't you? unless she manages to convert his sexuality which I very much doubt....
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