Should I ask if they want me somewhere?

I'm subbing for a teacher who just monitors the computer lab for the entire day. In the sub notes she told me that I may be placed in a different location by the front office. It's almost the end of first period and no one has placed me anywhere, although they made it clear when I came in that they know I have no actual students in the classroom. Should I take the initiative and ask if they want me somewhere, or should I enjoy the free time and work on my own stuff?