Active Sensing
MIDI's heartbeat — a byte sent over and over so the receiver notices when the cable gets pulled.
A Note On without a matching Note Off is a note that rings forever, which is exactly what happens when someone trips over the cable mid-chord. Active Sensing is status byte FE: a sender that uses it transmits it at least every 300 milliseconds whenever nothing else is going on. A receiver that has seen the byte once and then stops getting it assumes the link is dead and silences every note.
The feature is optional, and plenty of gear neither sends nor cares about it. The keyboards that do send it repay the favour by flooding every MIDI monitor with a steady stream of FE, so filtering it out is usually the first step in any debugging session. MIDI 2.0 and USB MIDI have their own ways of detecting a lost connection, which leaves the message largely a relic of the DIN-socket era.