質問: When you don't someone's name, how do you call his attention? That is, when you would say "Sir, could you maybe ..." or "ma'am, you dropped something ..."
回答: If you are working and they are a customer, then お[客]{きゃく}さま is a pretty normal way to refer to someone. Also in many contexts, お[兄]{にい}さん お[姉]{ねえ}さん お[父]{とう}さん お[母]{かあ}さん おじさん おばさん おじいさん おばあさん function as ways to address people when you don't know depending on the person's age. But perhaps the most common way to get someone's attention is すみません spoken in their direction.