Ghost? Can anyone explain?

Hi, My friend was babysitting for my daughter(9) and her granddaughter (6 months) . In the early evening my friend and the girls were downstairs and nobody else was in the house. In the room above the lounge, where the girls would be sleeping the baby moniter was plugged in. All of a sudden a noise came from it that sounded like someone in pain. My friend got a fright because the little red lights that go up and down on the monitor when it detects sound were flashing . This went on for about 30 seconds and my friend couldn't make out what the voice said. Although frightened, she went up to the room but there was nothing there. The window was closed the tv was not plugged in and no electrical appliances were on. When my friend came back down my daughter said to her "Don't worry, It was just a dead person" The baby also seemed unconcerned.Could this have happened because of a mobile,wii etc nearbyShe's a bit scared,and nervous at the best of times an explanation would help, thanks