trigger_multiples don't detect flags.
One way to detect the location of a flag is by using point_proximity_sensor, there's a [ame="http://forums.tf2maps.net/showthread.php?t=538"]tutorial and prefab here[/ame].
Another way would be to have a trigger_multiple around the flag that changes the flag carrier's targetname using !activator and AddOutput. A filter_activator_name could then detect if a trigger is activated by a flag carrier.