My family members do various things, not necessarily at the same time - but for roughly
for the same time each time. If a bicyclist typically goes for a 2 hour ride and they're gone for 4 hours, I'm going to start pacing. If the day is drawing to a close and they've been gone for 5 hours, yep, I'm calling LE if I'm not there to search myself. Biking is not without risk, all by itself. The idea that my family member could be lying in a ditch would be too much for me to not take some action. In the Morphew case, one would expect that the daughters would call upon Dad to go look for Mom - but he wasn't home, either. He wasn't that far away, according to his story (Denver - maybe 2.5 hours) but obviously, the daughters had a sense of urgency great enough that they called someone who was closer.
Perhaps they knew their mother frequently biked on Sundays, typically when it was cooler, in the morning (or something like that). They start calling her, no answer, by 2 pm. They know it's really unusual that she'd be gone for hours and hours on a bike ride. Whatever it is, the daughters became worried as the day drew to a close. Sounds like a typical, caring family action to me.
On Mother's Day, one can assume the daughters wanted to call their mom. Someone had had a text from her the day before, and the daughters were winding up a camping trip (apparently). I doubt theycalled just one time. I doubt that Suzanne typically biked all day long. At any rate, by late afternoon, they were worried enough to activate an elderly neighbor to go check.
It doesn't matter what my family (or yours) is like though. In the Morphew family, the daughters thought their mother's absence was significant enough for them to undertake action (we don't know all their actions obviously). We do know that until yesterday, they were kept out of the family home and that one of Suzanne's relatives thought they were "under protection." It will be interesting to see what happens next. One wonders why they didn't stay in the same nearby place as their dad, and why they did not physically try to undertake a search for their mother. If LE wanted them to stay away, there has to be a reason. Perhaps it's CoVid, perhaps not.
As it turns out, Suzanne is still missing, so the daughters' intuition about something being amiss was correct. I believe that's because a series of events on May 9-10 made them worried and uneasy. Even if Suzanne took bike rides at completely random times, her absence (from the daughters' point of view) was significant enough that they initiated a search for her.
What
@Dave F. said above was very apropos, IMO.