looked at boyfriends phone?

B
So there's a girl that my bf insists doesn't dm him, and this morning when he got up I looked at his phone and I see a dm from this girl that says "oh yeah?" Which is a thing that sounds flirty to me... so I open and and the whole conversation is cleared up until that point. Do I bring it up to him? 
This is why I hate going through phones because I find things out and there's no other way to bring it up other than to say that you were snooping, but it's killing me that he's lying about it and what she says sounds like it's in response to something flirty 

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