<human>: Can you succinctly summarize the text in 1-3 sentences?

Before I begin I have never snooped nor will snoop through my gf's phone. On our way home tonight I happened to glance over to the passenger side mirror and saw the reflection of her phone while she was selecting and deleting specific messages.

The message conversation was from one of her work friends (40f). I'm not accusing her of anything and truly didn't mean to watch her phone. At times I have been uncomfortable with how they interact, but its mostly just paranoia and a little bit of jealousy. 

What is the best way to bring this up? She doesn't know I saw and I don't want it to lead to an argument. I really wish I didn't see her do that and for sake of my sanity need to know why they were deleted.
<bot>: Saw my gf delete specific messages from her friend (f) need advice on how to bring it up and talk about it like adults.