So enemy bases are split between different spawners. Each spawner has its own units and scouts. So whenever a scout discovers you, you don't stop them and they run all the way back, the corresponding spawn will now see you as "discovered". But other spawners will still send out scouts until you are either discovered or enough time passes and you kill enough scouts before they can return. Eventually all the spawners ( with some small exceptions ) will send out units to attack you. Although, sometimes a scout will lose their way and remain in a spot somewhere in the map, instead of going back. On top of that, if you approach a spawner for the first time, there will usually be a bunch of units that just stay there instead of attacking with other units. And they will also sometimes give yellow ping, since the game counts some of them as scouts and they were already spawned in when you first entered the map.