Turns out the size of the IKEA Tradfri control unit is just slightly higher than the gap under the front of an IBM AT. If you push the front of the Tradfri unit, say, cos you lean against it without realising it. Then the Tradfri box slides under the AT, which presses the button on the face of the Tradfri, which turns the lights on, and starts cycling through the colours.
It wasn't some malicious hacker playing with my lights. It was just the Tradfri box being squeezed under a 30 yr old PC.
3/3