Please help. I snooped a look at my wifes phone and found messages from her possible ex bf.

We are both in our 40s and have been together for nearly 10 years. She was acting strange the other day. We were sitting right next to each other in public and she received a message. She hid her phone with her hand from me so I could not see. She looked for 2 seconds at the phone and put it away. I thought it was odd. I asked what it was and she said "oh nothing".

She was acting a bit strange around me so I regrettably checked her phone while she was in the shower. She had a message from a guy who knew her 20 years ago. I did not have time to read the messages but I did see it was mostly a catch up chat both ways. I do remember reading she did say my husband is not the jealous type. He did write how good she looked. I wish I had more time to read the rest as I am not sure of the context of the conversation. It looks like he tracked her down through facebook and she has friended him.

We have a great marriage with good lines of communication so this is very worrying.

How do I approach this?