At the offices of one of my customers, the printers are extra-friendly. Whenever I arrive or leave, they all greet me with a small concert.
I suspect that connecting or disconnecting USB devices from my laptop (such as the mouse dongle), CUPS is sending a broadcast over the network, querying printers of their status. This seems to wake up all Epson printers and force them to do a small self-check. This certainly needs more research.