It's a bug. I get it randomly when I crash.

If you crashed then your session may still be active, wait a while and it should time out.

I've had it happen a lot recently but ever since I exit out chrome and steam completely before launching the game (SC version) it hasn't happened.

My game randomly crashed, and on the "Retry in safemode"-screen this message showed up. Is this a bug, or have I been hacked or something? What should I do?

"You have been signed out because your account is logged in elsewhere"