“I can’t find my cell phone,” my husband murmured as he paced from room to room.

“Where did you use it last?” I asked, reaching for the home phone to dial his number—a tried and true way to locate a lost phone these days.

But no luck.

We retraced our steps, searched every corner, even called the restaurant and our friend’s car from the night before. Still nothing. With his business trip coming up, the missing phone felt like a small crisis.

The next morning, as we pulled into the chu...