Not just tacit -- I used to work at a factory, where as an engineer I was exempt. But I was on 12 hour shifts, 3 on, 4 off, 4 on, 3 off. Except that if there's a meeting, you stay an extra hour or come in an hour early. Or if there's an incident, you stay until everything is cleaned up. And if there's a meeting on your off day? You come in anyway. You get paged in the middle of the day / night when you're supposed to be asleep? You call in, and if things are bad enough you come in. Nothing tacit about it. If you didn't do those things, you were getting fired.