There were people who left Japan during Sakoku, particularly sailors who were caught up in storms and deposited on other coasts, but the key point is they couldn't return. The Japanese government refused to take back anyone who'd been abroad. The first time I know of that a returning sailor was allowed back was John Manjiro and his two companions who returned to their native Tosa in 1851.

One famous Japanese exile, whose story may interest you, was a clerk by the name of Dembei who was shipwrecked in Kamchatka in the early 18th century, and brought to St. Petersburg to tell Peter the Great all he knew about Japan, and to teach Japanese to Russian students.

It's quite possible that some people did secretly manage to go out of Japan and return, but they would have kept their mouths shut about it. One very late example casts a light on how this could have been managed: in 1834, three Japanese castaways landed on the shore of present-day Washington State's Olympic Peninsula. The Japanese govt. refused to take back the trio, who went on to jobs in Macau and Canton. Eventually, one of them, Otokichi took a job on a British ship that in 1849 entered Uraga Bay to try to open up relations with Japan. This was just a temporary visit, but to avoid problems with the Japanese authorities, Otokichi pretended to be Chinese, and said he'd learned Japanese from his father, a Chinese merchant who worked in the Nagasaki trade.

This brings me to the second question. The outside world knew precisely what was going on with Japan. Sakoku was not a wall between Japan and the larger world, but a strict regulation of where trading was to take place. Westerners today largely know about the small Dutch trading post of Dejima at Nagasaki, but Chinese traders were also welcome and very numerous in the port of Nagasaki, where they had a large quarter of the city. Korean trade took place at the island of Tsuushima which is halfway between Japan and Korea on the shortest route between the two countries. In the north, the Matsumae clan handled trade with the Ainu on the frontier of Japanese control. And, last but not least, the Ryukyu Kingdom (today's Okinawa) was officially an independent vassal state to both Japan and China. In practice, the Ryukyu Kingdom was under the domination of Japan's Satsuma domain, and Satsuma traded with China through the polite fiction of the Ryukyu Kingdom doing the trading.

So, Asian outsiders would have certainly known that there was an absence of Japanese travelers outside Japan, would have known why, and could have gone to Nagasaki themselves if they were employed by a Chinese trading firm.