Am I being naive?

So my partner of 5 years is always on his phone lately, we could be out together and he’ll always be texting, whenever I ask who it is he says it’s his ‘friend from work who’s going through some shit right now’
In the 5 years we’ve been together he has never given me a reason to doubt him.
But something isn’t sitting right with me, and I know it’s wrong but I unlocked his phone whilst he was sleeping and they have exchanged 256 messages to each other in the space of a week!
Some of the messages are general chit chat, others are like him telling her that he’ll bring her some perfumes/his aftershave to try out to work, for some context she said he smelt nice and he told her to use his aftershave.
Other messages they’ve been planning a day out together, and some he’s been giving her guy advice.
They both mention me by name in some of the messages so I know he’s told her about me.
Do you think they’re just friends or do you think this could potentially be something more?
Also I know that they had a drunken kiss at their work night out because he told me and I let it go.
Please help me..
Am I being naive?
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