The Stalker perk, as currently described:

"You gain increased sneak attack damage against hostiles that are investigating you, a distraction device, or a corpse you've left behind. +200% Sneak Attack Damage"

Currently, it seems that the extra sneak attack damage only applies when an enemy is investigating you (the first condition). The damage bonus does not apply when investigating a distraction device or corpse. When investigating a distraction device or body, the enemy does not have the purple stalker icon above their heads, indicating that it's not activating anyway. Regardless, the below numbers were a test conducted on one singular enemy, reloading between tests and the shot conducted from the same place, with the same weapon, so as to minimize interference from other sources. 

Stalker Perk, Enemy Not Investigating Anything: 424

Stalker Perk, Enemy Investigating Distraction Device: 424

Stalker Perk, Enemy Investigating Body: 424 (shot was taken as soon as the enemy started moving towards the body after noticing it)

Stalker Perk, Enemy Investigating Player: 706

I have this same issue. The icon show up when they investigate me but not with the distraction device or when they investigate bodies.