Having been through this, it does sound suspicious. And I knew anyway - my intuition told me. It's easy for this to happen and for people to start living separate lives when both busy with children, work etc. I wouldn't snoop, personally, as it's pretty low behaviour but talking to him calmly about it may be better...
In my case, after my husband dumped the woman, she came round to my house and told me he'd been having an affair with her. It was when my daughter was a baby and was down to the fact that I was more absorbed in her than him! Men do need a lot of attention I think and some of them aren't mature enough to realise that your energies have to be shared with the children too, not just them!
Good luck.