Is he still in love with her?

A few days ago my partner asked me to reply to a message from his sister via Facebook. While I was scrolling through to find her message I saw that he'd messaged his ex girlfriend.

It was an innocent enough message and just said Hi but he's been asking a couple of mutual friends about his ex lately too.

Apart from this we have a good relationship and this is the first sign somethings wrong. My best friend said I was being paranoid and I'd end up ruining the relationship. Am I just being paranoid about it? or could he still have feelings for her?