If a line were to be so faint that a given tech wasnt sure how to interpret it (or even end up interpreting it as a 'non-negative') the sample would undergo the confirmation GC/MS just to be sure.

Even if an untrained tech were to misinterpret a faint line as a "non-negative" and the sample was referred to the GC/MS for confirmation, the sample would come back as a negative.

As far as your pic goes, I can see the line clearly - and thats without "blowing up" the pic or using magnifying devices. Its a negative.

A properly trained tech would utilize this rule of thumb: If it can be photocopied, its a line.