This is actually a little tricky
.
You get the start of a flash when an enemy ENTERS the radar range. Same the flashing stops, once the last one LEAVES the radar range.
This means: if someone is already INSIDE Radar range when you enter the arena, it will not flash.
Same problem with players that get disconnected while being the only one inside radar (=flashing) range... the base will not stop flashing until someone else reenters and leaves the flashing range.
Also, a heavily damaged base will stop flashing. I never could pinpoint a certain amount of damage/specific structure that needs down for that to happen yet. Maybe a levelled town (ie no more population "on watch")? But im sure that Hitech could shed more light on that.