Spooky downlighters

Electrical transients "spikes" on your mains might be one cause. Have you noticed the lights coming on coincident with a motor turning on, perhaps?

If the downlight is a 12 volt halogen connected to an electronic as opposed to a traditional iron cored transformer, the dimmer has to be trailing edge not leading edge.

Does the dimmer switch have an "off" position, with an audible "click"?Is the light switched "off" or merely turned down to the lowest setting when this occurs?Do the lights come on full bright, or at reduced brightness?

More info is required to help you.Could be a dirty potentiometer, bad switch, bad circuit board,leaky capacitor, etc.

We see quite a bit of this where "tones" are sent down electric supply cables, for example to control off peak water heaters. These happenings usually come about at recurrent times. Some days of the week and some times have bigger effects. The tones can come be heard in poor quality sound amplifiers as a series of buzzes, and have been seen to trigger older style theatrical lighting systems. Tones are called Zelweiger after the name of the equipment used to read them in consumer boards.

What sort of dimmer controls? Do they have infrared remote control? Are they just normal manual switches? I need more info to work with. Manufacturer? Domestic/industrial? Tungsten/Halogen? Do you have multiple installations all doing this at the same time? Is it random or at some particular time of day night?

It helps to know exactly what is what here.