How did Sorrento not know he was still logged in?

The high end haptic suits and VR interface are portrayed as being very true to life in the movie - slightly less so in the novel. In-universe, it's meant to be close enough that Sorrento couldn't tell the difference as long as he didn't experience anything too different from what he would normally expect in that situation.

Current tech couldn't do it, but there has been enough advance before the movie for it to be hard for a user to tell the difference while in the Oasis.

The timing was the key to this scheme. As Aech explains to Sho, they spliced their feed into his rig just as he was exiting the OASIS.

SHO: So he really doesn't know he's in OASIS?

PARZIVAL: This is what Sorrento should be seeing. But this... This is what we're making him see.

PARZIVAL: We grabbed him in transit just as he was logging out of the OASIS. So, he thinks he's back in his office in the real world but... actually he's here, in the identical office that Aech built.

SHO: So, you basically just remotely hacked his rig!

AECH: A fixed rig like Sorrento's is easy to locate, hard to hack.

PARZIVAL: Unless he's stupid enough to leave his password lying around.

The key to making this work is that Parzival removed Sorrento's goggles. If Sorrento had reached up to remove his googles - at any time - it would have broken the scene that the heroes were projecting.

Since Sorrento isn't given a chance to take his suit off, everything he "touches" he feels through the suit, and he won't be able to tell the difference between the suit faking the touch - like the gun Daito prods him with - or a real touch, like his chair. (These suits are notably higher fidelity than depicted in the book, as shown when Parzival and Sorrento are fighting and their blows are hard enough to hurt.)

It probably didn't hurt that Daito was waving a gun in Sorrento's face, and he's basically a coward so this would be a great distraction. It isn't until he actually sees evidence of something that's not real that he begins to doubt the scene.