Four-twenty is the global shorthand for weed, and the reason everybody passes around is that 420 was the Los Angeles, or maybe California, police radio code for a marijuana bust. It has the ring of insider knowledge. Cops used it, stoners adopted it ironically, and now it is on bumper stickers and at 4:20pm everywhere. The police-code origin makes it feel official, almost forbidden.
There is no police code 420 for marijuana, in California or anywhere else. Penal code section 420 is actually about obstructing entry onto public land. The real story is gloriously mundane: in 1971, a group of teenagers at San Rafael High School in California, who called themselves the Waldos, met at 4:20pm by a statue to go looking for an abandoned cannabis crop. The crop was never found. The time stuck as their private code, spread through their connections to the Grateful Dead scene, and went worldwide. Time magazine tracked the Waldos down, complete with old postmarked letters proving the timeline. The mystery you imagined was five bored kids and a school statue.