YSK in an incident involving a casualty in public with a lot of people around, help them watch over their wallet and phone or any valuables they are wearing, or confirm it with the person they are with.
from /r/YouShouldKnow
YSKは、大勢の人がいる公共の場で負傷者が出た場合、財布や携帯電話、身につけている貴重品に注意するよう手助けするか、一緒にいる人に確認します。