My ex is hiding his new girlfriend?

So I went for dinner with my ex a few weeks ago, just for a catch up. I knew he had been seeing someone, but I don't know if he knew that I knew. Anyways throughout the dinner he had kind of mentioned if we wanted to try things again. We have gone out a couple times since then just casually, and he still hasn't brought her up. Even when I know they are hanging out he just says that he is with his friends. Why is he hiding her?
My ex is hiding his new girlfriend?
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