The best kind of "screwing around" is screwing around in plain sight. Once, I had a fresh grad in my team and I had to get him up to speed. I noticed he was taking more than the usual time to do basic tasks, yet whenever I passed by his desk, he had three terminals open and had c++ code here and there and compilation errors on another window. Two months later, he would move to a different company. Turns out that he was just practicing his programming skills, and all of this time I assumed he was working on assigned tasks.
EDIT: spelling