One thing I've wondered about is IR fluorescence photography.

I've seen some examples in document forensics where a page that looks blank (or at least the ink is unrecognizably smudged) because of water exposure is completely legible with an infrared photo illuminated by UV.

I suspect there must be a hidden world only visible in IR and UV (and long-wave IR, e.g. "thermal").

This is really cool -- pedantically, I've always thought "full spectrum" is actually misleading from a traditional photographic sense. Like IR + visible light + UV != full spectrum. I'd love to see post-processed imagery of every-day life through an extended view of broader EM energy (similar to astrophotography)... like what does a city scene look like with x-rays and microwaves included?

Side note: have always loved this image https://imgur.com/NZjWfWT of rainbows with UV and IR visible.