They never did that when I was younger. Though now that I am older, it happens every so often.
I woke up at around 9:00 am one morning (sister still asleep) and I knew my mom was planning on going to Wal*Mart that day. I asked when she was planning on leaving and she told me she already went...and came back! lol. My mom is such a morning person rofl...who goes to Wal*Mart at like 7-8 in the morning!?
This past week I went to my piano lesson. When I got back home I saw my sister stirring our dinner on the stove. I looked around the house and asked her were our parents were and she said, "I have been wondering the same thing for the past 30 minutes!" lol...I laughed and she told me she was in her room and the foyer door closed. She went out a little later and noticed two adults were missing, heh. She was keeping up with the dinner so that it wouldn't burn and felt lucky to have come out to notice they had left, otherwise the dinner would have been rather icky later! lol.
I looked out our living room window and saw then walking casually down the street pointing at our house and talking. I just laughed and waved to them as they passed by...hoping they would noticed I had returned home lol.
There have been times where I'd be out with my friends and return to empty house or, such as you, wake up to an empty house. But my parents are in no way negligent, heh. I mean, I am going to be 18 in November, my sister is over 16 and it isn't AS necessary to leave notes when they will be gone no more than say 1-2 hours at most. Usually they will tell me when and where they are going though.