WHen I make any modifications in seattle.master, I see that all the authenticated users are able to see the changes. However, when I am an anonymous user, I do not see any changes, it seems like seattle.master is never used for anonymous users. The problem is I could not find which master page is used for anonymous users. I even tried to modify all the master pages in SP Designer 2013 one by one (Yea brute force:)), but nothing changed. Is there anyone that can give me a clue about this?
Thanks.
I directed to the master page gallery myweb/_layouts/15/start.aspx#/_catalogs/masterpage/Forms/allitems.aspx, then I checked out seattle.master, then immediately checked in. While checking in, I selected v3.0(publish). The problem is solved like that. Thanks @Serv for helping me to get the solution.