The Apple Watch includes a useful “ping” tool that you can use to trackdown a temporarily misplaced iPhone. As the name suggests, this will make the iPhone emit a loud submarine-radar-like ping noise.
Just swipe up from your watch face to the Glances, then swipe right so you can see the Airplane/Do Not Disturb/Alarm icons. Then tap the iPhone icon at the bottom left.
What’s not obvious is that if you TAP AND HOLD this icon, rather than just tap, not only will you hear the ping but the iPhone’s camera flash will also illuminate a few times.
Obviously this is most useful if the iPhone has been placed face down (not a bad habit to get into) but in a dark room you should also see the light even if the iPhone is face-up.
Nice! Apple really does go the extra mile.