Sybill Trelawney does not know when she does actual prophecies.
"She does not know — and I think it would be unwise to enlighten her — that she made the prophecy about you and Voldemort, you see."
(Half-Blood Prince, Chapter 19, Elf Tails)
In fact, she's quite "out of herself" in a trance:
Relieved, Harry got up, picked up his bag and turned to go, but then a loud, harsh voice spoke behind him.
"IT WILL HAPPEN TONIGHT."
Harry wheeled around. Professor Trelawney had gone rigid in her armchair; her eyes were unfocused and her mouth sagging.
"S-- sorry?" said Harry.
But Professor Trelawney didn't seem to hear him. Her eyes started to roll. Harry sat there in a panic. She looked as though she was about to have some sort of seizure. […]
Professor Trelawney's head fell forward onto her chest. She made a grunting sort of noise. Harry sat there, staring at her. Then, quite suddenly, Professor Trelawney's head snapped up again.
"I'm so sorry, dear boy," she said dreamily, "the heat of the day, you know... I drifted off for a moment...."
(Prisoner of Azkaban, Chapter 16, Professor Trelawney's prediction - emphasis mine)
Snape was listening through the door when Trelawney made the prophecy about Harry and Voldemort and heard the first part of the prophecy. But he was discovered and did not hear the end.
"As you and your friends found out to your cost, and I to mine that night, it [The Hog’s Head] is a place where it is never safe to assume you are not being overheard. Of course, I had not dreamed, when I set out to meet Sibyll Trelawney, that I would hear anything worth overhearing. My — our — one stroke of good fortune was that the eavesdropper was detected only a short way into the prophecy and thrown from the building.”
“So he only heard... ?”
“He heard only the first part.
(Order of the Phoenix, Chapter 37, The lost Prophecy)
So surely, Trelawney being in a "trance" was not conscious of what happened (for her it was kind of like as if she had been sleeping).
But when she describes the scene to Harry, she seems to remember that Snape was apprehended and came into the room:
"I remember I was starting to feel a little odd, I had not eaten much that day ... but then ..."
And now Harry was paying attention properly for the first time, for he knew what had happened then: Professor Trelawney had made the prophecy that had altered the course of his whole life, the prophecy about him and Voldemort.
"... but then we were rudely interrupted by Severus Snape!"
"Yes, there was a commotion outside the door and it flew open, and there was that rather uncouth barman standing with Snape, who was waffling about having come the wrong way up the stairs, although I'm afraid that I myself rather thought he had been apprehended eavesdropping on my interview with Dumbledore--you see, he himself was seeking a job at the time, and no doubt hoped to pick up tips!"
(Half-Blood Prince, Chapter 25, The Seer overheard)