[AC Question]

My girlfriend is talking a texting daily how could I find the hidden app she's using?

[Tim1954]

Sounds like you need to resolve your trust issues there mate. I wouldn't snoop if I were you, it's not nice, and you may find things you don't like!
Talk to the girl my friend...

That said, if she's using your phone, you can look in apps... Show system apps and you will see every app in the device.
Or she could be using stock messaging app and deleting when finished. I do that too, not to hide anything, just like to be tidy and uncluttered.