The advice you rec'd is spot on. We also travel with 5 dogs and can only add we do the same as others have recommended regarding closing the blinds and turning on the radio or tv. One other tip that hasn't been mentioned is to take your pup for a long walk to tire him/her out before you leave.
Start your pup's training gradually, beginning as suggested by just being outside while she/he's inside, hang around and listen. Once your pup has master that, then leave for 30 mins., later extend that time period to an hour. We don't leave our dogs alone for more than 3 hours, and on the rare occasions we'd exceed that time we hire somebody to come walk them and make sure they're ok.
If you know who left the note go over to visit them, apologize for the disturbance, thank them for bringing this to your attention, explain your pup is in the early stages of training and your goal is for him/her to be a good neighbor.