left (behind)

You probably are correct yoong, however I would find the sentence satisfactory.

Behind (in this instance) means in a place, state, or stage already passed

I left my keys (in a place already passed) behind. I'm happy if you wish to expand on the information conveyed by attaching "in the Office".

I left my keys in the office.
Your suggestion conveysall the information using less words



Both examples are acceptable

#2 is more acceptable

Thanks, Dadpad.