They run their own detective agency, and receive a mysterious package with a walkie-talkie in it.

"You realize, I trust," he said, "that the mysterious package contained the security."

A mysterious package shows up at her boyfriend's workplace, killing him and six other people.

Jondalar saw the mysterious package among her other things.

Then there was a gag about the police receiving a mysterious package.

She assumed Seven would take her mysterious package there.

I saw your mysterious little package, watched you slip off on your secret trip.

He enjoyed the anticipation her mysterious packages had caused, and being in on the surprise.

He receives a mysterious package, inside of which is the Gamebook.

Because the mysterious package had no return address, I wondered who might have sent it.