Disgruntled employees act out in lots of ways. A guy I knew who hated his boss “played” on his work computer all day. The computer was strictly for constructing company graphics. But he installed all kinds of games and wasted lots of time. His boss never knew he blew off half the day.
Company computers are obviously company owned, making it legally possible for your boss to spy on you. Employers can also figure out whom you’re speaking to on your company owned or sanctioned phone and for how long