A sentry holds a place; a patrol holds a path. The difference is that what you play against a patrol is not position but timing.
Behaviour
A patrol follows a closed route written into the level data. On each enemy phase it advances one edge along that route, and the direction it faces is the direction it moves. On every node it stops at, its line of sight runs — and that sight is lethal.
The route never reverses; it is a closed loop. Which makes a patrol entirely predictable: you can count how many turns until it is anywhere.
How you read it
The route is marked on the ground — footprints, a wheel rut, a worn path. With the threat highlight on, the next step is marked separately, so you never have to count in your head.
Its facing is always its direction of travel, which means a patrol's back is safe every single turn.
Taught in
Taught in1-4
What this level teaches is waiting: sometimes the best move is to watch the patrol go past for a turn. There is no "pass" command in the game — you wait by stepping back and forth, and that costs two moves.
What answers it
Three things: count the rhythm and slip through the gap, step into a hiding spot and let the turn go by, or break it off its route with a bell stone. The third is not in your hands before act 2.