Yeah Europa is very promising along with Titan which has a shit ton of natural gas but that would probably be redundant by the time we had the ability to extract it.
I wouldn't be surprised, There's probably a lot of top secret tech the government is hiding, I mean take the SR-71 instance that was developed in the 60's and the public didn't know it existed until the 90's .
Also Some Asteroids have a lot of platinum, and a quite a few things can be done with platinum if it was more common, so that would be very useful too.