This happened to me as well. Working there for 5 years. First to arrive, last to leave. Constantly on-call without being payed. Worked on weekends, and so on. One day I left exactly on time, 6PM. Later that day, I received a text from my boss saying that It was unprofessional to leave at the exact hour. I didn't reply. Next day, first to arrive and sat in his Office waiting for him. I quit on the spot, gave him all my work tools, access codes, documentation, and just left.