This depends much on WHERE that person belongs to. For example, if he belonged to some far flung African country, the immediate changes he would notice, would be very minute. However if he used to be an inhabitant of the areas which now happen to be suburbs of Manhattan, Tokyo, Beijing or any other megapolis then our character is in for a world of immediate shocks.
This also depends on the age and gender of the character, the age effect being more prominent than the gender effect. Young (18–35) people would perceive changes far more quickly and in more detail than aged characters. And if the character is a young female, she would probably detect things far quicker than a male, due to the on-the-guard psychology of women of most cultures during 1000 AD.
Let's assume Mr. Kagwa (age 25, gender male) belongs to some far flung African country and he walks out of the woods. It is winter season and the time is evening.
Mr. Kagwa Walks Out Of The Woods
He heaved a sigh as he walked out of the woods. Safe, finally! There was hardly a chance of finding a hyena or a lion in the woods, but wildcats and leopards were a deadly threat. He stretched and looked south. There was smoke far away, near the horizon. It was rather strange. There was no village in that direction! None that he knew of, at least. Rather surprised, he had no time to walk in that direction and investigate. Night was dangerous!
He took the trail leading to his village. After walking for a couple hundred yards - don't use meters here- the trail suddenly ended! He looked around and rubbed his eyes a few times and looked hard. Yes, there was no trail! He had walked that trail since his childhood but it was there no more. Quite perplexed, Kagwa shook his head and continued walking the path. Trail or no trail, he had to reach home.
As we see, Kagwa notices only two oddities: smoke in an unusual place and the absence of the trail leading to his home.
Now let's compare that to the case of Duddley, a 22 years old English boy (man by his standards) who happens to walk out of the woods some 30 miles from London. It is summer and the time is afternoon. The sky is clear.
He walked out of the woods and stopped. There was a strange object laying on the ground in front of him. It was shiny and appeared metallic. Prudently, he proceeded and picked it up. It was indeed metallic, its weight was a clear indication. He examined it closely. It was shiny and hollow inside. -Duddley has found a piece of metal pipe- He carefully placed it back. It could be dangerous! He then resumed his walk.
There was something strange but he could not tell exactly what. He looked around closely. Everything looked the same, but there was still a feeling of something amiss. His heartbeat increased slightly and he started walking carefully, watching his every step.
Suddenly he saw something above and in front of him. He stopped and gazed. It was very high in the air but he could still tell it was large! Far larger than any bird he had seen. He examined it closely and suddenly noticed something horrible: it was not flapping its wings! That, and there was a trail of white smoke behind it. He stopped in his tracks and stared in horror as the giant bird flew past him, never flapping its enormous wings.
He then realized what was amiss in his surroundings. It was the smell! There was a slight, but consistent smell of smoke in the air. And the noise. Drrrrrrrrrrrrrrrr... It was faint, but he could easily tell it was coming from above. It was coming from the bird!
His heart thumped in his chest as he ghastly looked around. He quickly perceived this was not the place he had known. It looked similar, but it was not the same!
One metal pipe. One airplane. The pollution and the background noise. Duddley known within 5 minutes something is severely wrong around him.