[quote=@Prince of Seraphs]
<Snipped quote by KabenSaal>

That's the embodiment of the theory that reality only exists as long as it is observed. The moment observation ceases so does reality.
[/quote]

Observation comes under the 'until' part. Because, unless Gren has some spare eyes I didn't see in his CS, he can't observe someone behind him until he turns around. Therefore, 'until he turns around' is a valid statement.