Why Is He Still Hiding Things From Me?

Here we are, been through the mill and back with his infidelity, and we are no longer together. Now that we don't see each other on a daily basis, there are still times that I have to see him. We have a child together and I see him when I have to. It's funny though because every time I see him, he acts like he's hiding something. To this day, he still hides the fact that there is or ever was "another woman". Why does he feel the need to be sneaky, even after it's over? Is he trying to hide her from me, so I won't go and ask her any questions? Is he just trying to make me believe that I was only imagining her, her phone calls and his visits to her? Did he think that her leaving messages on his cell phone were invisible to my ears? What ever the case, I am not going to obsess about it, after all, if he had wanted to be with me, he would have tried his darndest to work things out.