On my Android phones I have always used a folder with a .nomedia file in combination with QuickPic to store password protected pictures.
I've recently got a new Galaxy S6 and have continued with this. I plug my phone into my Windows 7 PC via USB, created the folder with .nomedia file in and copied my pictures over. I then go into Quickpic, choose 'Show Hidden' and my pictures show. All good.
However, coming back some time later, I go to Quickpic and do the same, and nothing shows. Plugging back into PC via USB and the folder itself is no longer present.
The first time this happened, I had had the phone about 10 days and the files had been there, but then suddenly they were not. Yesterday I readded some files and they were present - however coming back less than 24 hours later, they have gone again.
I've read some questions that sound similar, but the general case is that people have a .nomedia file by accident. I don't, and have been using this method for several years without issues. The files are literally not there any more, not showing within android file system or when attached to PC via USB.
Restarting makes no difference, but again, why would it when the files have been physically deleted.
So, what is deleting my files?