A dinosaur's input here:
I'd guess that 3 vs 5 was due to the sensor
suite on a drone system.
As something that has to interact with the world, an effective drone (or whatever) wouldn't be limited to a simple camera setup, but redundant laser/radar rangefinders linked with machine vision.
The other thresholds might have been errata'd in to make gameplay less taxing, but it makes reasonable sense and also makes drones a lot more unnerving (and a logical purpose for a security-minded company)
OR 3 = Joe Schlub munching on a cruller and missing the improved invis team as they grimly run past the hidden security cameras. (Also great for storytelling)
OR 5 = That mean looking security drone that's scanning your area.
Additional fun, like ultrasound, isn't really Improved Invis, it's Silence or some bastardized "Look, I've got awesome skillz and a spell that could possibly fake out the system if I roll enough successes and manage to sweet-talk the GM - oh, I shouldn't have said that out loud" spell setup.
-Tir.
PS:
Oh, and we've always played Unconcious = Sustained Spell down, unless said spell is anchored or sustained by a foci for such.
Think of magic as a series of tubes, the mage can't suck the magic out of the tube if they're unconcious, but if they've set up a pump and turned the damn thing on, well, there you go.
PPS:
I just love being able to use the "series of tubes" analogy.