If you really want to/ need to know where they sre (worried they might hurt themselves or are in danger…) and the phone is on your plan, call the phone company and ask where they are. They can tell you the location of the cell towers nearest, if not the GPS location. If the phone is actually off the phone company can tell you the same information for when the phone turned off. Phone company may balk at telling you, but will if you push… talk to supervisors…

Some of your comments sound like anxiety, depression like issues may be present. This would increase the need to know their location. When you are back with them, try to have them talk with the bishop, Church Services and/ or a therapist (counselor, physiatrist, psychologist…) depending on the severity and what they are willing to do and what works.

Show love. Show you care. Without causing too much guilt show you were worried. Try to talk with them. If you think there are serious self harm potential issues ask these questions at a time there is a chance to have calm discussion:
Have you hurt yourself?
Have you thought about hurting yourself, including using a blade to cut lines on arms or elsewhere or other self harm?
Have you thought about suicide? Just the idea comes up? Actually pondered it? Planned it? Tried it?
Depending on the answers, take appropriate steps