Everybody has a set of thresholds beyond which they WON'T go back and get their cell. Mine on weekends when I'm going out for local errands is about 1/2 mile. For longer weekend trips, maybe 3 miles. For a work day when I'm needing the phone to help somebody in the field, probably 10 miles. For overnight trips offroad, I'd never leave it because it's a mission-critical piece of safety gear and on the go/no-go checklist if I'm alone.
My wife on the other hand, can go a day or two not checking voicemails or texts. Ten relatives could die and she wouldn't know. So I'm the one to remind her to pick her phone off the charger and carry it, IF ONLY TO CALL ME OR AAA if she had a car problem she couldn't fix. She would never turn around to get her phone because she's never notice it wasn't with her.