I'm not sure about mindfulness meditation specifically or what exactly that even refers to, but some people use a mindfulness bell that is rung randomly a few times a day for exactly that purpose. One old form of mindfulness practice is to use breath changing from in to out or out to in as moments of mindfulness. See Centering in Zen Flesh Zen Bones translated by Paul Reps (from a document predating Buddhism):
There is also a kind of meditation practice called "just note gone" where the idea is to start giving absense of stimulation a distinct notice (by noticing when something you had been perceiving is no longer there). I think most sit down and meditate practices tend to focus on that kind of thing (feeling the absense of stimulation as a distinct thing) via different methods.
https://shivayashiva.wordpress.com/2014/01/12/centering-prac...
There is also a kind of meditation practice called "just note gone" where the idea is to start giving absense of stimulation a distinct notice (by noticing when something you had been perceiving is no longer there). I think most sit down and meditate practices tend to focus on that kind of thing (feeling the absense of stimulation as a distinct thing) via different methods.