The Photos Library has pictures with filenames like this:
88B20509-DEF0-479E-939A-B68B32A84E7B.jpeg
These are unique file names that Photos uses internally for your pictures, because names like IMG_0123 probably occur many times. You're never meant to see these file names-- they're just for internal use. These pictures all appear in the Library view of Photos, but with their normal filenames. There shouldn't be any pictures with names like this that you don't see in Photos.
If this isn't what you think you've been seeing, then you can do a Finder search for the images on your computer. Here I've put a "." in the search field,
because all files have a dot in them. Then I hit the "+" box to add more filters, and I chose "Kind_is_image_jpeg" You could pick all instead of jpeg if you want less common types.
If you select a picture you can press command-space to see a "Quick Look" at the picture. At the bottom of the window you can see the path to that picture.
There are a lot of pictures on a Mac-- it could take awhile to go through them all. You can add further filters to reduce the quantity by clicking on "+" again.
Let us know if we helped or if you need (or you think that we need) some clarification...
