Can not see it today at the right upper corner where it usually was, am I missing something?
The forums sign in is tied in with Market so there isn’t really a sign out option unless you sign out on Market.
click your photo in the top right below Tuts+. opens a menu dropdown, right at the bottom says logout 
Thank you @KingDog for reply! But today I went to forums being logged in my marketplace dashboard and was needed to sign in to forums specifically as usual (there always was a button at the top, but now I did it through random post “reply” button). Then I tested it a few times and it still seems the forum loggage works separately, no matter am I signed to market or not.
Perhaps, it’s only my side issue though.
Aha found it, thank you Gareth!
Not working, here. Had to hit the reply button to sign in.
Yes, did the same.