tmp folder?

I have a number of WP sites and one of those is running 3.5.2 (yep, I need to update it) but what’s strange is that this appears to be the only one that has a tmp folder at WordPress root and it seems to random files such as “sess_0b65057fecc8cb2ef20d768c365eab17”

Not sure what this is and why it keeps on getting written to? timthumb is being used – would that have anything to do with it?