This ad in today's Register-Guard caught my eye:
If I can read between the lines they're basically saying, "Do you have any of those funny little cylinders laying around that you have no idea what do to with? We can decode them!"
So that's what it's come to. Film is such an obscure medium that the phrase mystery film is somewhat repetitive. To most people any film is mystery film. Since few people are likely to shoot it anymore, the developing industry has readjusted its marketing aim to cover all the rolls shot and lost years ago. Perhaps they've recently been discovered under the sofa cushions or in the back of the glovebox. If so they need a decoder. Probably some fun stuff on there.
And at $1 per roll who can resist? That's pretty darned cheap. If I had any color 35 laying around I'd know where to take it. As for b/w, it's still cheaper to develop inhouse. It's all beautiful mystery film, sometimes even after it's been decoded.