Hidden video on iPhone
It would seem that it could only be in one of two places (and perhaps a third in a stretch) if you recorded it with the built in camera app. 


Photos -> Albums -> Videos (or Slo-mo)
Photos -> Albums -> Recently Deleted


As I'm sure you are aware, you can also see videos under "Photos" (left most tab), or under Albums -> Camera Roll. I suspect that you checked there, though. 

If you tap and hold on a video, you can choose "hide". That will hide it from places other than under Albums so the first option above should still work even if you have inadvertently hidden the video. 

There is also an extremely remote chance that it is in the Videos app. But, you'd have to have to specifically put it there using iTunes as a go-between, so again, it is highly unlikely.

By the way, my phone says that I have 119 videos and the camera app's videos album shows 119.