Should i message or leave it alone?

So last night my boyfriend got this new drone he’s been wanting for a while. When he gets it all set up we go outside to use it, he hands me his phone and asks if I can take a video on snap so he can show his friend Sean. He was taking a minute so I sat down and put his phone between my legs. I guess where it didn’t lock it I accidentally swiped to the screen where all his convos are. One down at the bottom said 2-3 wks ago and it was girl names Alexandria. I unfortunately wasn’t treated so well in the past and have horrible anxiety when it comes to stuff like this. I added the girl on snap and then asked him about it before deciding to go any further, he tells me they’re just friends and they don’t even talk regularly. I don’t know if I should just let go or message her to see if he’s telling the truth. I do trust him but with my past (being extremely horrible and damaging) it’s hard with certain things. I just wanted some advice and opinions on what I should do.

Editing this in

1.) yes it was an accident his lock button doesn’t work on his phone so he has to use the assist thing on iPhone. My lock button works it’s not something I think about everyday so when I set it down it didn’t lock.

2.) he usually is very open with me when girls snap him. He usually goes ahead and tells me “hey whoever snapped me today” I never asked him to do that he started that on his own.

Finally.) the above statement is why I found it weird and got me even more anxious. Because he didn’t tell me like he usually does.