The Marauder/Sentinel stealth really isn't stealth, it's a very short term ability usable for both escape or offense. You're not going around with it undetected for any significant period of time.
All classes which have long term (modal) stealth abilities (Agent, Smuggler, Shadow, Assassin) have access to an 'in combat' stealth, the ability to engage their stealth ability while actively engaged in combat. This is a fairly recent change we've hinted at a number of times. Players that follow the stealth centered skill tree in their class (e.g. Infiltration) also have the ability to further improve this ability.
#12678