Is it possible for photos to be saved without knowledge

Today I was looking through my camera roll and between taking a picture of my dinner yesterday night and some pics before sleeping the same day, there are 40 photos that have been inserted into my cameral roll between these two sets of photos.

I had an irrational fear that somebody elses photos had been saved to my phone unintentionally due to a bug in photostream or similar, but then again I have photostream turned off.

The pictures are of varying quality (quite low) and seem to be tourism photos. E.g. ski resort, bars and ocean cruises interseded with lots of photos of bars, restaurants...

Can anyone explain this? It seems like a potential security issue. I've never heard of photos being able to download themselves onto my phone without permission, nor did a google search pull up anything.

I know for sure it couldn't have been me to downloaded those pics, because I hadn't used the iPhone at all today...