I have a basic LG cell phone which is not smart and can't flip, but it is sneaky. To use the secret compartment, slide open the battery cover. There it is. You just exposed it.
It's only big enough to hold one US dollar bill folded 2 times. I'll often keep an emergency 100 in there. Also I caretake vacation rentals & condo units and have to deal with many combination locks. Sometimes I need the list of codes and I don't like to keep a picture of the codes incase some else decides to look though my phone.
The main thing is to keep the paper underneath the camera eye and speaker part. And close the cover right. Some paper notes I make barely fit and the cover bulges making it slightly more apparent.
Thanks, that's it.