I've leveled up an Operative and I've never seen this 'super-stealth-detection' people are claiming all droids have.
1. Bosses have insane stealth detection, and you can tell because the stealthed person will get a symbol over their head telling them they're about to be detected. A few seconds later the boss usually aggros.
2. Some (note: SOME) droids have a stealth scan ability (note: ABILITY they can activate, not a passive trait) that looks exactly like the BH's stealth scan. Yes if you stealth anywhere near that thing you will be knocked out of stealth immediately, but it's a clearly visible animation that is easily avoided.
If you stealth up to a non-boss droid that does not have a stealth-scan ability (or has just used it and the effect has faded), their stealth detection chance is the same as any other mob.
There are droids with the same passive stealth detection as bosses.
As to which have this and which don't, I can't say, because there's no reason for me or anyone to stealth around elite droids in flashpoints to try and find out which is which. There's no benefit. Nothing to gain by being stealthed around a droid, might pull them, can't out of combat CC...what's the point of risking it unless you're a concealment op or a middle tree sin?
However, I'm pretty sure the beefgate mob in hammer station has them because I've seen that mob get pulled by shadows and/or smuggs in stealth from long range when I was leveling a Van tank.