Many people, particularly those which may be in a dangerous situation, are familiar with the Morse code for SOS. Im not, but itd probably be on my reading list before taking a three-hour tour. And its not even that useful for signaling distress, requiring a functioning emitter and a nearby eavesdropping
Instead, we need a pattern that is easily learned and reproduced, most likely using a dozen or so points, some equidistant to another, to create a shape that can be represented on the earths surface with contrast. It will be unique enough to exist very infrequently by chance, but generic enough to be memorable even for children.
The pattern should be reproducible at any scale with only a fixed-length measuring device and the ability to alter the landscapes contrast, whether by drawing, carving, holes, or placing rocks or manmade objects. Think of the grade school pointy S as an ideal balance of simple to remember yet geometrically complex. Sadly, its origination without an emergency context makes it an inappropriate choice and potentially widespread in aerially-viewed graffiti. Perhaps another letter, symbol, or icon in the same style?
The ability to pick such a pattern out of satellite imagery already exists. The EURion constellation is used worldwide in banknotes to prevent reproduction and its search invoked with nearly every photocopy made. A known distance and orientation of points is sufficient for modern image processing software to pick it out of the noisiest backgrounds with trivial computing power.
Human review for false positives would be the easy part. Response to prank or artistic depiction eliciting rescue are up to the bounding governments policies but would likely consume fewer resources than driving out to the sites of hundreds of 911 butt-dials.
As satellite imagery is updated at an increasing rate by both governments and corporations, the viability of passively searching as its released and still managing to find survivors increases. Going by the recency of Google Maps images of my area, Wilson wouldve still been no more than a volleyball to Tom Hanks by the time he was scooped up.