Mick Gordon did some fun hidden spectrogram imagery in the Doom 2016 soundtrack.
He talks about that and plenty of other cool stuff in his talk at the 2017 GDC conference. One of my favorite conference talks ever, he did so much cool experimentation to get the sounds he used on the soundtrack, and watching his talk is one of those moments where you really get to see a master of his craft let loose and explain his process.
https://imgur.com/sRe6Ypv
Aphex twin did something similar, but this is more playful in my opinion.