I don't own a blacklight, any suggestions on a cheap one? I tried passing a light source through a blue filter, not sure if that's how it works, but didn't see anything.

Wouldn't blue channel expose anything on the print if there was something hidden? For example it works to make the yellow dot pattern pop out in the scan.

Frame I disposed of a while ago as I broke it apart to see if anything was hidden inside of it, so that's gone.

Thanks for the suggestion btw, I've thought about it as well.