Would it be nervy to say this?

You use something that can hit someone if they were to come into its path right next to someone's bag that holds their important belongings. When there's temporarily no activity is when they decide to quickly get them. While they're picking them up is when you tell/taunt them to get out of the way or they'd get hurt.

Their stuff was there first. You carelessly/nonchalantly picked that spot altho there were empty areas available. They waited until there was no movement, so they assumed it was finally safe to get their thing(s). Plus, they don't have all day or else they'd be late for their next destination. Even if your intention was good to *warn* them, would it be nervy to tell them that in this case-even if said it playfully?