Thats why its smart to clear your setting or cookies or whatever it is, i do it all the time when i use a comp that isn't my own.
i can't remember if Newgrounds has that check box that says keep me logged in since i haven't logged off for the past like 3+ months i think XD and haven't needed to type my info in for awhile. if there is one just don't check it and usually it logs you off when you close browser window i think at least in most cases.
Sonucais
You probably just left your account logged in somewhere and some one started playing the game, that's it and did not even notice an user was logged in.
PJ-Olzow
Thats probably what it is, though I haven't signed in to this account in a long time. No damage was done and I thought it would be a good excuse to make a post about :D