All major players know exactly where each other’s silos are. The real question is, do all of them have missiles in them?
A missile costs the same as a missile, but a hole in the ground is vastly cheaper. If you can entice an adversary to use a missile to bomb your hole in the ground, you can cheaply soak up counter force strikes.
My impression is that US missile silos are located mostly in sparsely populated areas of the Great Plains and Rocky Mountains, where there are not a lot of locals to overhear the garage door opening.
The caps to missile silos are several feet thick and made from concrete and steel. From what I remember, when the door opens it's pushed to the side by rockets. So it's a little more dramatic than a garage door.
They can open the doors slowly via some hydraulic method. I do not suspect during tests they use the rocket opening option which can open the door in two seconds.
Was there a large audible sound in the scene? I've seen the movie more than a few times as a kid but don't recall loud mechanical sounds of the missile doors. If it was a drill, no doubt they would had routinely opened the doors on the regular in the past, and at the bare minimum, they should have done due maintenance to ensure silky smooth and squeek free opening of said doors.