I've never thought that the burglar "brought his own burglar alarm", ie, he spent all that time in the home writing the note solely to have it on the steps so as to cause much commotion should a parent happen to be wandering about in the middle of the night and discover JonBenet's absence. The hours of remaining in the house are incongruent with the desire for a few moments of advance warning when really no such warning would have been all that valuable to him or all that necessary.

Whatever reason the note was left for, it does not seem to me to have been created for any sort of 'kidnaping' or 'early warning system'. I think it was left for the same reason that garotte was embedded so deeply in her throat: it was fun!