By Yik Lam and Jessie Pang HONG KONG, June 29 (Reuters) - As documentary filmmaker Kiwi Chow walked through a pedestrian tunnel in Hong Kong on a recent day, he spotted a team of cleaners scraping off glue left by illegal ads and scrubbing the walls clean with mops. It reminded him of the day he became an activist during the pro-democracy protests that galvanised this Chinese-ruled city in 2019. Back then, he had gathered with his young son and some friends to cover another tunnel wall with Post-it notes scribbled with political messages and drawings. Strangers joined in. "I was very happy. It was the first time I was an organiser," he said.