Ah-ha! We know what it is. The Orlando Sentinel was first to break the news that the mysterious device that shut down the entire Disney World Transportation Bus System was a harmless tire pressure monitor with remote monitoring capabilities. This device was installed by Disney as part of a test program back in the 90s on just one bus.
So, as I suggested earlier, this was something Disney itself installed and the current team had no knowledge of. Occam’s Razor suggested that as the most likely answer and it was. Mystery solved.