Do people say 一下 as a very colloquial "just a sec"?
My thought is: 


  He probably said '等一下' but you didn't hear the word '等' due to the noisy  environment, or the speaker's volume was too low when he spoke


The only reason to use only '一下' (a sec') instead of the complete phrase '等一下' (wait a sec') is he knew you were waiting, and your expression told him that you were asking "How long do I have to wait?" in your mind, so he just gave you a short answer instead of a whole sentence, based on the presumption that you knew what '一下' was referring to. (referring to the length of time you have to wait-- a question that wasn't actually asked but implied in your manner)

Imagine a dog owner was eating and his dog looked at the food intensely, the dog owner might simply say "不" instead of a full sentence "狗不能吃人类的食物, 所以不能给你"  because he presumed the dog would understand what the word "不" referring to. ( he imagined the dog was asking "can I have some?" - answer was "不" )

People give short answers all the time in conversation with the presumption of both of you knew what it meant