This is almost pure speculation, but I suspect it is a quest-related "phantom" life sign.
I have seen this in a number of places (as a sneaky sort, Aura Whisper in invaluable). Notably, I have more than once seen an entire crowd of such life signs in impossible spaces in small, heavily-populated cells where quest events take place (such as the Ragged Flagon - Cistern). These life signs have a tendency to appear and vanish as quest events occur.
While we don't yet have the tools to properly investigate, I suspect that they are the result of quest scripts "storing" actors (quest-related NPCs) outside of the cell geometry until they are needed. This is a technique used in other games (including previous TES games) to ensure than an actor is fully loaded before use (to prevent disk lag) or to pre-run scripts prior to interacting with the player.
The resultant life signs are therefore "real" in the sense that there is an actual NPC at that spot, but also "fake", as that the NPC isn't supposed to be visible to the user, due to not yet being "present".