The cape has only 3 or 4 exit routes. Bourne bridge, Sagamore bridge or by boat or plane. Should make finding them a little easier.
If it wasn’t planned, there should be less to worry about, but I imagine that if it was planned and they left the cape before their absence was noted, or had a boat pick up arranged.
We used to hitchhike from RI to Ptown in high school. Not a care in the world. So dumb!