Using "beyond" with nothing following behind

The use of a directional preposition without an object is not uncommon where the object is readily understood from the context.

The following is a line from a common translation of the Odyssey

The opposite point seems more a tongue of land

you’d touch with a good bowshot, at the narrows.

A great wild fig, a shaggy mass of leaves,

grows on it, and Charybdis lurks below

to swallow down the dark sea tide.

The preposition below has no object following it, but refers back to the tongue of land . . . at the narrows.

Your usage is clear in its meaning. It also does not suffer from the awkwardness meant to be avoided by the the "rule" that a preposition is something you should not end a sentence with.

Typically, hotels are located in high-traffic areas rather than on dark mountainsides. Thus, upon reading “I gazed out of the hotel window. The mountain lay quietly in the dark...” I supposed the narrator to be looking out and up at a mountain's outline in the dark, with a city's lights visible in the distance beyond the mountain but off to one side, as lights behind the mountain would not be visible. Had it read “gazed out of the chalet window” or “gazed out of the cabin window” or “lay quietly below us in the dark” I'd have supposed differently. In short, your usage of beyond is not wrong, but the passage as a whole doesn't communicate the same situation as the picture does.