I started practicing today on my Oscar Schmidt Baritone Uke and the strings were buzzing like crazy. This hasn't happened for a long time. I live in SoCal and temps were mid 60s and RH was 35. So, turned on my room humidifier and set my uke aside and jumped on UU to search for buzzing. Found this Thread and am sure glad I did. Much thanks to the OP for posting this, will definitely save for future use. I hadn't done any checks listed in this post and was just getting ready to do some of them. The room was about 45% RH at that time, pulled out the uke and...no buzzing!!!
So, environment is definitely a cause of buzzing -- add it to the list.