You probably already know that you can use your Apple Watch to ping your iPhone as long as they're close to each other. (Open Control Center on your Watch and tap on the Ping icon, the button with the iPhone and sound waves coming out of it.) Your iPhone will beep a few times; you then repeat the process until you find your phone.
But wait, there's more.
As explained by a Reddit post from late 2015 I stumbled across by accident last week, if you long-press on the Ping icon the iPhone will not only play a sound, but the flash will blink several times to help you find your iPhone in the dark.
A feature that was likely designed with accessibility users in mind is useful in more than one scenario.
Your iPhone's screen will need to be off for this to work, and as you can see it's bright enough to grab your attention even in a well-lit room.