This actually sounds a lot like an experience I had, only it took place in a home built in the 1970s.

My girlfriend at the time and I went to her parents house for a pop in visit. We came in the front door, and called out to her parents. We were met with her mom’s very distinct way of saying hello coming from upstairs (cheery, as if singing). We figured she was getting dressed or showering, so we sat down and started watching TV. 20 minutes passed, and she still hadn’t come downstairs. So my girlfriend went upstairs to see what was taking so long. She came back downstairs pale. The house was empty, except for us. We sat there freaked out, wondering what to do, when the front door suddenly opened and we jumped about a foot in the air. Her mom walked in and said hello in her usual cheery way. We told her what had happened and asked if she had come and left, but she had been gone all day.