Enemies can detect you even if they're not looking

[CandySquirrel16]

When in stealth, the alert meter increases if the enemy's facing right at you, but also increases if the enemy's facing right or left.

[EA_David]

Hey, do you have a clip of this. It sounds like you may have still been in the cone of vision for enemy NPCs, but if you have a video of aberrant behaviour that'd be very useful. 

Cheers.