Possible meanings for "returned" are 1) he returned to thinking. Alternate translation: "I thought again and and I saw something meaningless" or 2) he returned to seeing something meaningless. Alternate translation: "Again I saw something meaningless"
This refers to things that are done on earth. See how you translated this in [Ecclesiastes 1:3](../01/03.md). Alternate translation: "on the earth" (See: [[rc://en/ta/man/translate/figs-idiom]])