The Jester and Umbran are very knowledgeable fellows when it comes to 3.5, the Jester especially when it comes to spells.
All they're suggesting is that they, in their wisdom, haven't seen such a spell in any published work.
They offered suggestions as to spells that would accommodate your idea, without perfectly fitting, because they're the best possibilities without making a homebrewed spell.
Hiding in someone's shadow, as it moves, and remaining completely undetectable by any form of observation would be highly irregular for a 3.x spell. I can think of no monster, no spell, and no power that would allow that.
The only, albeit a distant second, ideas that I can come up with would be Invisibility, combined with Nondetection, combined with a high Move Silently check. Ghost Form could, along with Invisibility, also mimic what you want, but you're still not hiding within a shadow cast by another person.
Any attempt to shift into another plane (other than the Ethereal Plane) would result in not being able to see what was happening on the Material Plane, so going into the Plane of Shadow wouldn't really help. The Ethereal Plane could be used to accommodate what you want, but then you are still detectable by anyone who can see into it, along with all the denizens of the Ethereal Plane.
I posit that you are hesitant to ask questions on forums (this one, in particular) because you are known for becoming belligerent when told you are wrong/what you are looking for doesn't exist/questioned further.