depends on which model. The newer printers go into a "sleep" cycle which allows some controls on waking for self maintenance. It sounds like the heads are sealing when going into sleep. I know that many times when I've turned it off, the printer makes no noise ... so the heads are already sealed.
I leave my printer on all the time now (first Epson I've done this with in a long time) , and I hear it wake itself up every once in a while, does something then goes to sleep. I don't know if it's spitting a little ink out, but it reseals the heads. I haven't quite figured out exactly what it's doing, there are 3 different settings involved, and none of those times seem to be part of this. For example I have the printer set to auto shut off after 4 hours, yet it never turns it self off, but I have come in and it's been "asleep" ... everything off but the blue power light.
The older printers maybe more of an issue, but I would think it would have to be off for an extended periods of time. If you lost a whole channel it sounds like your damper might have been stuck, and a little ink loosened it back up. Often the best course is to wait a short period of time after cleaning that channel so the fresh ink can loosen that damper back up.