how to listen for "off target" taps on iOS in most efficient way?

If I present a target on the screen for player to tap, what's the most efficient way to know if they've tapped the screen but missed the target?

By efficiency I mean absolute lightest on processor etc.

There are three "targets" on the screen. The desired target, a pause button and a menu button. However any taps anywhere else I want to know about and then generate an error message to the player. But the game is designed to be light and fast. So I don't want to use a bunch of objects around the targets to be this checking facility. Unless that's the only way...

For touch interfaces on iOS, I use colliders and raycasts. This is fairly inexpensive. If you are doing 2D, Sphere and Box colliders tend to do the job.

Theres a lot of information on the Unity Website regarding these.