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 is always a closed loop. Which makes a patrol entirely predictable: you can count how many turns until it is anywhere.
A back-and-forth patrol is not a separate rule. It looks as if it turns back when it reaches the ends, but in the level data it is written as a closed loop with stops (for example a, b, c, b). The route line shows an arrow at the ends, so you see the turning point in advance.
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 make a mistake counting.
Its facing is always its direction of travel, which means a patrol's back is safe every single turn.
Taught in
Taught in1-32-7
The first level introduces the closed route, the second the back-and-forth patrol. The shared lesson 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
There are four ways: count the rhythm and slip through the gap, step into a hiding spot and let the turn go by, break it off its route with a bell stone, or use the takedown from behind. The direction a patrol faces is the direction it moves; coming from the node it is heading to is coming from the front, whereas approaching from behind or from the side is not. You do not have the bell stone in your hands before act 2.
If a bell stone pulls a patrol off its route, its progress along the route freezes. When the patrol returns to duty it does not make up the turns it missed; it rejoins at the nearest occurrence on the route of the node it returned to, going forward from the frozen point. So a back-and-forth patrol, too, never turns around invisibly.