Hmmm... gonna go out on a limb here.
In my experience the meter only shows the relative threat of an area. The needle doesn't move! If its at the low end (light yellow), the threat is low, and it will take a while for the Agent to show up. If its at the high end (red), the threat is high, and the agent is gonna be on you like SNAP! I presume that the threat level corresponds to yer players level, vs. the level of the are you are in. I.e. being level twenty and in Westview is low threat, being level twenty and in Stanford is a high threat.
Could be completely wrong, but I've never seen one of those meters move, so im guessing thats how it works.
And yes, on the original subject, it is the Data Scrambler 0.4's that cause lvl255 agents to spawn while datamining. In the regular districts of the city, they just drop dead, but in datamine they will chase you until you leave. And no, as Zudrag stated, you will not see your Agent threat meter light up. Further more, there may be some lag between tapping the node, and the Agent starting to spawn, so it may SEEM like you hadn't even tapped the node yet, but thats because its coming from your previous tap. My suggestion is to use a Data Extractor 3.0. Sure they have sucky range, but they pull down more info than the Scrambler, and yer not getting hassled by lvl255's. (The only thing I don't know is how they work with the new Datamine luggables, as the description in the Update notes only mentions them multiplying the info level of the Scrambler.) So if you have to use a Scrambler, and you hear the sound of an agent Spawning, just start beelining to an exit, only thing you can do.
