A regular occurrence in the Golden Age was the arrival of a package at some mobster's office or hotel room:
It was always a signal to the crook that Batman was on his tail:
Batman sent it as a message to a judge:
It even popped up on a cover:
Any more modern examples out there?
Update: Here's one from Batman #121 ( February 1959):
Making a Splash: Kane and Anderson's Batgirl
2 days ago