A valet switch is part of most modern
car alarm systems.
Suppose you need to lend your car to a friend, or take it to a full-service car wash. Maybe even go someplace so fancy they actually have valets. (Hence the name...) It's not usually practical to give these people the remote to your car alarm -- they're typically kinda big, at least on the scale of things that go in your pants pocket -- but at the same time you don't want the poor bastards to have to listen to your car's pointless screaming. (Many car alarms have a cutoff either to the ignition or the fuel pump, so they'd be screwed, and your car might be damaged as well.)
Enter the valet switch. It's a tiny switch (rarely more than a couple centimeters), tucked away in some unobtrusive location (most typically under the car's dashboard on the driver's side, but occasionally in the glove compartment, or even under the hood in the fusebox). Flip the switch, and the blinkenlight usually goes from blinking to solid, and the alarm is disabled until you flip the switch back.
This node inspired by Suntrup Used Cars, who sold me my car without showing me where the valet switch was. I've spent the last month looking for the tiny little bugger.