Probably nothing. My guess would be that this is something to do with the Unix backend of OS X, and it might well be a byproduct of the app you're using.
It's been a while since I've needed to do this, so you'll probably want to doublecheck the exact syntax of the command, but give this a shot.
Open up a terminal window, and type in (without quotes) "rm -f ~/Trashes/*"
Basically, I just forget where it is the Trash folder links to in the filesystem. I'm pretty sure it's ~/Trashes but I can't remember for certain. If I had a Mac handy, I'd just probe the filesystem myself, but I don't at the moment.
Malware is possible, but highly unlikely on a Mac. There are a few proof of concept malware attack vectors for Safari, and by extension the Dashboard, but so far as I know, no one has ever actively exploited them.
When I empty my trash there remains a string $ $ $ o $ $ . o $
as shown ,it is empty and apparently does nothing ,but it bugs me to know its there
I have emptied the trash with Cocktail, but it continues to happen
could it be malware