Have an exit button maybe? Could not exit after the picture was taken. it goes back to camera mode and you are stuck.
Had to go to feedback window, was then able to exit the program.
I finally found out how this works. Windows 10 Mobile lets you swipe in from the bottom of the screen to show the OS buttons.
I agree. It sucks that the navigation bar (with back button and home button) is hidden in the Windows 10 Mobile app version. Please change this! Do not hide the navigation bar in Office Lens on Windows 10 Mobile!