What I generally use is a system of cycle waypoints with a large placement radius, which makes the patrols very, very random.
If you wanted a more random system, make four patrols for every one you actually want, put the leaders
probability of presence bar down to 25%, and make the rest of the group's
condition of presence being that their leader is alive.
What happens then is you get a whole random patrol, generally you'll get one out of the four you put down