On most illustrations of a spellcasting the somatic component means doodling glowing magical glyphs in the air —Warcraft game and movie exemplify this, but the PHB itself has also images of it. (I am not sure what the magical lines/traceries/glyphs/runes are called or if they have ever been given a name by WotC, Pratchett or other... ???)
From what I understand the magic blue lines are not in 5e, PF or 3.5. A wizard does not give his position away in a dark room —bar for the Verbal chanting. A 3.5 caster with Silent Spell, Still Spell and Eschew Materials feats has to just do a Sleight Of Hand, Bluff or Perform to hide the pointing/mental targeting.
But was this always the case? Even in 3.0, AD&D or some splatbook? Or does the fluff anywhere mention the other characters' seeing of a picture being drawn in the air?
EDIT. More than a justification of it in the rules, I am after an insight into why it isn't there. It seems odd to me that the magical lines is how most picture spellcasting, yet it completely absent in the main rules. I fully agree that it's a required artistic licence otherwise the illustrations would be of a bunch of bearded men in a dress making pouty faces. Yet the aspects of spellcasting that would have a constraint on the caster, i.e. chanting (Verbal) and hand gestures (Somatic), are there.
I was wondering whether it was there in AD&D —which I have never played— and taken out as it seems strange it.