Shut down/Restart your phone. I still have this issue every now and then. When it loads back up, check your gallery and all of your pictures should be back. I think one of the apps I have installed causes this or it's another general storage issue on the DINC.
Hope this helps.
I can't figure out why photos are disappearing from Gallery as well, but they are not lost. Go into your file manager and go to: emmc - - DCIM--100MEDIA. You will find your photos there.
I'm actually curious to see if anyone else is having this problem, I just got a refurb and when I read this post I figured I'd check it out. Low and behold it happens to me too.