I don't think there's anything wrong with talking to someone of the opposite sex. My best friend is a guy. We had a history too but nothing physical. Most of my friends are guys and my husband is more than okay with that. He has plenty of female friends too. A lot of times though, if there's someone new, he'll forget to tell me but if I ask about it, there's no issue.
I believe you did the right thing by telling your husband you were talking to the old friend. I do the same thing.
I do want to know if your husband was trying to hide the female or was it in the open for you to find. If he wasn't trying to hide it, you shouldn't trip.
A lot of times, my husband will talk to someone and not delete the messages because he knows I'm going to play on his facebook later anyways. Same with me, my facebook is always logged in and he has the passwords, there's nothing to hide.
personally I find it odd that he would talk to a random woman on the bus and add eachother of facebook to communicate. If it's business related I could understand but just for some chit chat?...yea I don't know. But no he shouldn't be mad at you.