Since I was a kid, my immediate family had a little tune of four notes that we whistled to get each other's attention. This "family whistle" was particularly useful in crowds. Mom might be looking around, not seeing me, and I could whistle, letting her know which way to look, without yelling, "Hey MOM!" We could also find each other in the grocery store without looking down every single aisle, and again, without obnoxiously shouting.
In decades, I've never come across anyone else whose family does this. Until I was at Machu Picchu.
I was coming down Wayna Picchu, the big mountain that's always in the background of photos of the ruins, and overheard a guy telling the rest of his group "...'s our family whistle," after which he blew out his family's four notes. A girl commented on how easy it was to hear, then they got too far away to make out. I was so excited to find out that it wasn't just us, it was a phenomenon. If I'd stumbled on one other family whistle, there must be even more. I pulled out my phone and recorded myself whistling the notes so I wouldn't forget. I just found that recording (along with the horrible-quality sounds of street vendors, a kid begging on the bus, and the mating calls of blue-footed boobies) and it reminded me to investigate.
So how many more families are out there with their own family whistles? I just did a Google search, and found almost 2000 references. Many refer to a Holocaust memoir titled "The Family Whistle." Knowing how my family uses its whistle to locate each other, I can only imagine that its role in this book is gut-wrenching.
But there are many happier references too. Someone mentions in a forum about family words that just about every family in Mexico has a whistle. A blogger stumbled upon the concept in Target and isn't sure what to think. A columnist reminisces on the lost tradition. The BBC even has a page with recordings of half a dozen such whistles. (Though I don't have room for Real Player on this old computer, so I haven't listened.)
I'm tempted to speculate, as I find others have, that cell phones have probably replaced this tradition, but haven't you been talking to someone by phone who was in view, but couldn't figure out where you were? Wouldn't a quick whistle be simpler?