BWAHAHA! It took me a few times' worth of hearing the beeps of the person across the aisle's cell phone, but suddenly I realized what the signal was. The pattern was three short beeps, two long beeps, and three short beeps. Each time the phone sounded, its owner would flip the device open, read, then text a response.
At first, I thought, "It sounds almost like Morse code for 'SOS', except that the letter O is three dashes, not two."
Then, a memory from eons ago surfaced: two dashes is the way to render the letter M in Morse code. Once upon a time, in Hell-imentary school, my friends and I thought we might converse in class via Morse code. But we couldn't manage to signal each other without also annoying the teacher with the noise, so the experiment lasted only one day... but I digress.
Anyway, the moment I remembered the meaning of two dashes in Morse code, I realized that "... _ _ ..." means, "SMS". So the phone was actually saying "SMS" whenever it received an SMS. :o) Clever! I got a chuckle out of that.
Yeah, yeah, I know, I'm a nerd. So sue me. ;o)