Okay, please bear with me on this here...
Everyone here is making a single, fatal, assumption. That they were actually IN Shadar Logoth in the first place!
Let's look at a few inconsistancies.
First of all, Liah. He left her there to her fate WEEKS before. If Mashadar can kill so quickly, and leave no trace, how is it that Liah survived not only one night in Shadar Logoth, but many consecutive nights? Why didn't she leave? Find her way out somehow? Oh no... Liah was already dead, my friends.
Second, this stranger that helps Rand. I, too, believe it could have been Lews Therin. "How could that be," you ask? Simple... But please bear with me a little while longer...
Thirdly... Everyone is tripping up on the lack of use of any sort of the Power being used... True Power, saidin, saidar, or otherwise. Who says ANY of the Power was used to fake Sammael's death?
Remember, Rand was running after Sammael through the palace in Illian. Rand had already followed Sammael through reading the weaves of the gateways that Sammael had used previously, correct?
Okay, so imagine this scenario:
Sammael, being the master of defense that he is (who is also quite good at evaluating, planning, and scheming), knows that Rand is coming, and has a plan ready for use whenever he needs it. Rand, coming and triggering all of the wards around Illian brings Sammael to him. Sammael makes a statement about not wanting to trash what they're fighting over (I mean, who wants to fight over a rag doll, and end up ripping it in half just because both children are tugging on it so hard?)... To Rand, this fits with what Lews Therin has told him about Sammael, so he only grins, and (foolishly) follows Sammael through the gateway that Sammael had weaved.
Here's the trick though... Sammael weaved a gateway into Tel'aran'rhoid's version of Shadar Logoth. Rand andSammael fought in Tel'aran'rhoid , not in the real world. Liah was merely conjured up bySammael’s mind as a distraction (how convenient, eh?). As far as the "stranger that saved Rand" goes... That, indeed, was Lews Therin; conjured up by Rand's own mind, and dissapearing without a trace of the Power being used. (notice how what Rand did with the Power, the stranger damn near mirrored? Which is why when their balefire touched, it was such a shock.)
That, my friends, is why the Power couldn't be felt by Rand, both when the stranger left and when Sammael escaped.
You see, the "mist" that engulfed where Sammael was, was harmless. It was merely a normal mist, conjured up by Sammael's masterful control of Tel'aran'rhoid . Sammael pulled off a Houdini trick that fooled Rand into thinking he was dead, when he was actually just hiding in the harmless mists of his own mind. *smiles*
(For those of you that don't get it and need it spelled out... Rand thought that the mist was deadly... It wasn't... Sammael is alive.)
wotmania says: Hmmmmm... I cannot recall hearing anything like that before... It is certainly an interesting idea, though I am not sure how much I buy into it. I have said it before and I will say it again... Anything is possible with Jordan, and I feel like this is just the kind of twist he could throw at us...
I just would like to clarify the question weather Sammeal is dead or not.
He is really really dead!
In a chat with Mr. Jordan himself this question came up and take a look at his answer...
Question from Arsolos: It has been reported that you have confirmed that Sammael died at the end of Crown of Swords. Could you confirm that you have said this and elaborate on whether Rand was correct?
Robert Jordan: Mashadar killed Sammael. Sammael is toast!
and don“t take my word for it... take a looksie here:
http://www.cnn.com/chat/transcripts/2000/12/12/jordan/index.html
Sincerly,
Marcus


Gandalv the Gray
Aes Sedai of the Age of Legends 
Faithful servant of the Creator 
Bringer-back of Moiraine 
Decapitator of Myrddraal and their Kin 
the real car'a'carn
Sammael is dead