A guard WP is probably the best way to go. However, if you want more control over exactly what the squad does, use triggers type switch synchronised with waypoints.
Your riot squad has 6 waypoints
1) move
2) move
3) cycle (placed close to wp 1)
4) hold placed in area of trigger 1
5) hold " " " 2
6) hold " " " 3
trigger 1 is syncro'd with wp 3
" 2 " 4
" 3 " 5
The riot squad will patrol between 1) and 2) till one of the triggers is set off. As long as the triggers are type switch then the riot squad will go to the area of the trigger that has been fired. You will need to experiment a little with this idea to make it work properly for your specific case, but I hope you get my drift.
Of course the guard waypoints thing solves your problem use it, its much simpler.