Bf's ex-wife texts him and he hides it

I came home and pushed the home button on my bf's cell to see what time it was (my cell was dead, it was sincerely not in snooping. I just wondered how long I just spend in the car). It said his ex-wife's fisrt and last name @gmail.com..... I was like on I'll pretend I didn't see that and see if he tells me. I saw his face when he saw his phone, not even a shocked reaction. I later asked about his day, if anything out of the ordinary happened, ect... Giving him every optitunity to tell me. He didn't. I said so what did ur ex wife say? And he said, she said hi. And he deleted it. He showed me he deleted the message, but who knows what it said. What would u do? If he had told me I wouldn't have been mad at him. But he his it... I kinda feel in my gutt he does things behind my back. I think when I get my finances rt that I'm going to leave him.