I can’t find any mention of this movie trope:
In movies and TV, often westerns, the hero looks up and sees a clear glint on a mountain or a roof top: somebody is watching them with binoculars.
I don’t buy it. A glass surface (unlike a mirror which has a silver covering) reflects only 4% of the light.
And more, a binocular front lens is curved. This means that the reflection of the sun is very small and weak, and falls off with the square of the distance. It is nothing like a reflection in a hand mirror. (And if it was, it would have to be extremely carefully aimed to be visible to anybody, because the light beam is only as wide as the mirror.)