It probably is a browser filler. I really want to find a way to block that from happening on my site for other users also.
It can be really annoying !! In fact, when I log in to WA, I can't even see what I am typing because it turns my type white and the background yellow. I attached a screenshot of it...my id is actually typed in there. So I have to stop and click/drag over it to see what I have typed.