BBM.
For me, this comment supports the theory that it was a predator who lived in Nicole's building. If Nicole and Melissa went for a swim "all the time," then it would've been easy for him to observe patterns. He would've noticed that she would be allowed to take the elevator by herself on occasion.
Personally, I think it was a crime of opportunity. He'd had his eye on her for some time, and would entertain the fantasy but didn't think it would go much farther than that. Then one day he pushes the elevator button, the elevator opens, and there's Nicole all by herself in a swimsuit.
The suddenness of her disappearance, I think, reflects the suddenness with which the predator acted on this unplanned coincidence. It was pure impulse because there was no time. Here was his one chance being handed to him, and he knew it would never come again.
I don't fault the mother because the 80s were just a different time period. Nicole had done this "all the time" without incident, so why would that day have been any different? And often times people see their apartment building as an extension of their own home. They might not let their child go to school alone, but they might not see anything wrong with letting them check the mailbox or meet a friend in the lobby because they're still "inside."
Also, missing children as a phenomenon was only beginning to be recognized. I don't know about Canada, but for us in the United States, America's Most Wanted and Unsolved Mysteries were still 2–3 years away, so although abductions have always occurred, they weren't really being brought into your living room the way those shows did.