For one thing, Nancy was not someone who would just blow off plans and disappear. Her friends' experiences over time led them to understand how Nancy was, whether she was reliable or not, whether she was punctual or not. These friends were with Nancy a lot, talking to her if not every day, almost every day. That's the testimony.
Have you never gotten an intuitive feeling? I sure have a few times, and they've been downright spooky. One was the night I found out my brother had died--got a weird gut feeling something big and bad was happening, about 3 hrs before I got the call. And yes it was completely unexpected.
Donna Lopez got that feeling. Nancy's friends felt something was off that day. JA testimony is not that she was worried after 1 hr. She called the first time around 9:30 - 10am to see where Nancy was. By the time JA got really worried, Nancy had been gone for nearly 6 hrs and did not bother to call, leaving 2 young kids and not showing up for plans in the morning or the afternoon. Why do you think they checked hospitals in the area? In case NC had been injured during what they thought was a morning run. Brad told them Nancy left around 7am. JA call to NON emergency police number was after 1pm.
You can discount human intuition but it does exist and sometimes you simply can't ignore it.