The use of passive radiator speakers is (or was, back then...) quite common in high-end hifi systems, and the isobaric cabs (Orange, I believe, and a few others...) use 'em, too. Normally, a speaker's cone is braked by the connection to the amp, but when unplugged (or, it would appear in this case, disconnected...), that brake is no longer, and the cone becomes free to resonate with any passing frequencies. I'd suggest that this could explain the cone movement, and the sound emanating from the unit. The speaker has become a passive radiator, resonating at a specific frequency (or its harmonic...) that happens to be audible.
...
Or ghosts, of course.