You would say "He works hard behind the scenes" to mean he works hard where people don't see him, like at home or in a less visible role. The first sentence, "He works hard behind," is incomplete and doesn't convey the intended meaning. Have a great weekend too!

Correct grammar is "he works hard behind the SCENES" and the meaning of it is your second guess (behind the scenes = from unseen places).

In this context, "behind" is a preposition of place that always needs an object. It can also be a noun when you're talking about someone's backside, but it's clearly a preposition in this construction. "Behind the scenes" is a common idiom as Dan already mentioned.

"He works hard behind" doesn't make sense to me. "He works hard behind the scenes" means exactly what you think: he works hard when and where we don't see him. "Behind the scenes" is a standard phrase and "scenes" is usually given the plural form. The phrase literally refers to live theatre. You have a proscenium at the front of the stage. Behind it are usually several backdrops. And behind all the backdrops, stagehands are working hard, silently, moving scenery and props around.