Is there something to be worried about... all opinions welcome.

My bf asked me at the start of our relationship (almost 1 year ago) whether I'd have a problem with him keeping in contact and seeing an ex gf on a friends basis. I told him I had no problem with that so longs I was free to keep in contact and see an ex bf who I was still friends with. He wasn't happy about me seeing my ex bf so I told him he wasn't to see his ex gf. It wouldn't be fair to have a set of rules for him and a different set for me. So he agreed to no contacting or seeing exes. 
Recently I've noticed he hides his phone from my view when messages come in, he never leaves it unattended and has a passcode on. A few months ago a message came into his phone which he didn't hide very well. I seen quite clearly it was from the ex but he never opened it so I never seen the content. 
So the first opportunity I got to have a glance through his messages while he wasn't there I took. There was very recent messages from the ex gf but he had literally wiped all content from his phone except from the stuff he had sent/received that day. Basically I found nothing untoward however there wasn't anything except a few messages to look at. She said she felt lonely and he had replied by saying she could talk to him and he didn't mind. He was there for her just like he had told her before. She asked if I minded him being there for and speaking to her to which he said I had no problem with it and I knew!! 
Do you all think this seems a bit suspicious to you? I'm pissed he said he wasn't going to contact her but has done but I'd like to hear whether you guys think he's hiding something? 
I'd also like to add that we have a pretty awesome relationship. We see each other fairly regularly and have spoken about buying a house together and a future!! We are happy minus this little niggle I've got about the ex. I have asked him if he had heard from her or if he had contacted her to which he told me he had not spoken to her since before we got together!!!