Facebook messenger secret conversations.. cheating?

I feel bad for writing this because honestly i’ve never really had doubts in my marriage but lately, we have been a little distant and while I don’t really see him on his phone too often, I let my insecurities get the best of me and went through his phone. I didn’t find anything. But.. on Facebook messenger, under the secret conversations it says “first seen by this device: July 9th”

I’m wondering, what does that mean? I did ask him and he said he never has used it before. but about 2 months ago, the same thing happened. It said “first seen by this device” then has a date but I never seen any messages.

I remembered the iPad is hooked up to his messenger and it says that the iPad is enabled to see his secret conversations but the iPad is with me almost every day and it rarely ever goes off. So I’m confused on if I’m just stretching on this or if this means he’s talking to someone