They can. There are programs with which you can monitor (see what's on screen, read type-ins) countless number of computers on the firm's network at the same time in real time. Employer has the right to supervise employee's work at any time. You are at your work place, the firm is your employer's property, you can't refer on privacy of work while working for your employer, it's the same as if you would close yourself in the office and not let your employer in to see what you are doing.