Yeah, I can confirm that that works as well, and as you say it's a more efficient way of doing it.
It's also nice that the two radii can be different if desired, so you can remove the markers if the player is within 1k but detect troops in contact with enemy within 300m.
Running it every 1s makes for a fantastic flashing effect that really draws attention to the marker, and I like the fact that it can be active at multiple locations simultaneously. It could also be used as a FLOT indicator easily by changing the marker type and duration.

As I said, thanks a lot for this, it's going to add a lot to my scenario (which I really should post here somewhere if I ever stop fiddling with it!).