It is implied that secrets, vents, and armoury are KOS. Warden can remove these restrictions if they see fit, or add additional ones. Guards should not be out of sight of stack anyhow (following the ST is not a valid reason to be out of sight), so unless you're doing this in front of their nose, a problem shouldn't usually occur.
I don't really see an issue with how it's done currently, it doesn't make sense logically for ST to be allowed in armoury/vents by default (you wouldn't be able to do this on a free day, why would you be able to on an ST freeday).
The goal of ST (in a game sense from the CT perspective) is to remove threats to the round by giving "special treatment" to someone who could threaten the round in order to convince them not to. Why would that special treatment be "let's allow them in vents, secrets, and armoury so they can be even more threatening later on when our bribes don't work anymore!".