By RAW if Stealth-6 remains loaded and the other icons in a node do not succeed on the non-extended threshold 6 Observe in Detail matrix action to detect the hacker is the hacker able to be detected if he attacks a spider?
In other words does revealing ones self through attacking make them targetable or must they be seen first?
There are some very common misconceptions/misinterpretations in this post. First of all, I'm not sure where people got the idea that spotting a hiding icon is a Threshold test, but it isn't, it's Opposed:
If your target is running a Stealth program, the Matrix
Perception test becomes an Opposed Test, with the target rolling
Hacking + Stealth (or Firewall + Stealth for programs or nodes) as
the opposing dice pool. The hits from this test reduce your hits and
consequently the amount of information you get. If you garner no
net hits, the target is not invisible as such, but its icon has melded
into the background of data traffic, escaping your notice.
Second, (in bold) a failed attempt to spot an icon running Stealth doesn't mean they're
invisible just that they are
uninteresting. Now, I concede that by RAW there is no rule that directly states that Attacking an icon makes you easier to spot, but it does "just make sense." At the very least, the Spider would know they are under attack and set the node to Active Alert, which should give them a +4 to spot the intruder. However, given the fact that Stealth doesn't make you "invisible" just "blended into the background" I would say launching an attack pretty much nullifies the Stealth program.