This is actually on the film. I had it done at a shop but I did the scan myself on a flatbed. It showed up on the 4x6 test prints I got from the shop as well. It's faintly visible on a couple frames, but it's almost not noticeable. That's why I thought it was so strange that it was so heavily pronounced on this one frame (which thankfully isn't an important picture haha)
Could it have something to do with the processing they did?