404 pages coming from nowhere?

I have these 404 pages crop up on google, but the URLs are just long serial numbers after our index page?

If I investigate it, it says one of our valid pages are referring to it… but I have no idea how? We have over 1,400 vendors, so I don’t know how or why these ~80 pages have cropped up?

Could there perhaps have been an old website at this URL? Could these still be references from an old website?

We’ve always had this website domain, no previous owner. It’s very strange!