There is this:
Mining In Chaffee County, Colorado
MOO etc ...
It doesn't show every adit [entrance] or every attempt.
They are all over the place within a few mile radius.
Some would be good for hiding things, but I don't think they would be easy to get to if you were carrying those things. All the ones I've found that are sort of obscure require leaving well-defined trails and bushwhacking a bit. I don't think I could carry a person to the hidden away ones I've seen because they are on steeper terrain w/difficult footing. If you were really strong and had a buddy, you probably could but that seems super risky.
The easy-to-find ones are on more travelled trails, but you'd lose cover, and a few of those I've seen are mostly filled in to keep people out.
The big abandoned mine structures you can get to w/AWD or one of those ATV type vehicles, but I've stayed out of them because they are rickety as heck looking.
So - yes - there are lots of options around for that. I would assume (?) they looked at any obvious ones but maybe not the obscure ones because they are so hard to get to. I figure if I know about them they aren't super secret, so dogs would have picked up a trail. I've always, maybe incorrectly, assumed that they'd have tracked BM's scent along with SM's, right?
I think this would be one of those things that if you called the tip line and said "Hey, check the mines" it would be right up there in helpfulness with "Hey, did you check the phone records?"