Verified answer

"Sur" usually means "on".

As for "au Québec", it's just an idiom. (You often can't translate preposition exactly between languages.)

It's the same in some varieties of English: you aren't AT a place, but TO a place ("she's to the general store right now").

I think you can actually compare to English in this particular example. Since au is a contraction of à + le, the sentences with au actually contain the definite article "the."

Je mange au bureau = I eat at the office.

This sentence expresses a habit, or at least your plans for today. Which office is "the" office? Surely the one where you work...

You could probably come up with a few contexts where it would make sense to say, "I eat in an office" (dans un bureau)... but only a few. Perhaps if you were complaining that "you work in an office, you eat in an office, you feel like you spend your whole life in an office!". Or else in a situation like the one Bix mentioned, "I have a client meeting tomorrow, so I'll eat in an/some office I haven't been to before."

Je travaille dans un bureau = I work in an office.

The sentence means that your work place is an office, that physically speaking, the place where you work is in(side) an office or an office building.

Je travaille au bureau = I work at the office.

The sentence is perfectly correct, but it does not make a general statement about your workplace. Instead, we expect it to be part of an idea like, "I do my work at the office, not at home" or "I work all day at the office, so when I go out I don't want to talk shop!"

sur means on (like on top of)