I Have a Dragon Timer Question

For all camps if we know for certain when they were cleared our timers should reset, right? In other words, if we SEE the enemy kill baron we will see on the mini-map (and timer) when he respawns.

So, how come when I am fighting in lane and I see my opposing laner glow red (which means they just killed the dragon) my dragon timer isn't set? We have a clear visual on when it was killed...