Why would a girl hide the fact she has a BF?

Anonymous
There is a girl that always flirts with me and shows me pretty good signs of attraction, there are lots of them and they are all positive. I found out she has a boyfriend but she never mentions him so I decided to test her...
She was supposed to go on vacation with him but when I started talking to her about the vacation she said "I am going with (she paused for a few seconds) with my friends".

Why is this all happening?
Why would a girl hide the fact she has a BF?
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