My son has a 2011 Macbook Air. It was working fine on Friday morning. On Saturday morning, it had gone to a grey screen requesting a passcode, with four white boxes, of the type you would see if the machine had been put into Lost Mode. However, I didn't put it in lost mode (in fact, I didn't have "find my Mac" turned on for it at all). We are not aware of having set a passcode on this machine ever, and it's not responding to any of the codes we were likely to have used (for example, the code for the parental restrictions). We did buy this machine used, and while I thought everything was wiped when we bought it, maybe somehow the previous owner did this (but it seems unlikely, we've had it for over six months, and I would have noticed if there was another iCloud account active). I have high confidence that the previous owner was, in fact, the original owner and not selling shady goods to 10 year olds. This leads me to think it's actually been hacked. Is this something that is happening? And are there any good ways out of our problem?