dog whistle

"Keep your eyes and ears open. Stay alert for anything you might sense. Don't talk to anyone. If anyone approaches you, leave. If you see a bunch of bad guys showing up, start honking the horn and get out."
"Okay," she said. She looked a little pale.
I pulled a silver cylinder out of my pocket. "This is a hypersonic whistle. Mouse can hear it from a mile away. If we get in trouble, I'll blow it and he'll start barking about it. He'll face where we are. Try to get the car as close as you can."
"I'll have Mouse with me," she said, and looked considerably relieved.
I nodded. "Almost always better not to work alone."


White Night Chapter 34, Page 352-353