According to
this post, logging out on one device may or may not log you out on all devices. Specifically,
this message states that this design was an intentional decision.
I did a couple quick checks using 2 web browsers and a phone. Logging out on either web browser logged me out of the other browser. Logging out of browser A also logged me out of the phone (I didn't check if browser B logged me out of the phone due to lazyness). It appears that logging out on one device (i.e. whatever you're currently using to access the forum)
should log you out of the missing tablet. Please check my work though!
Best of luck recovering your tablet, or if that's not a realistic option, closing all the security holes the disappearance has caused.
ETA: I used a browser on the phone as well, so I didn't check any mobile app. That would probably be a good next step.