If its a really slow night at work and manager asks if you want to go home early and you say yes all the time

If they ask you and you didn't ask them I don't see why they would think that. They're probably asking you to save money on payroll

Make hay while the sun shines.

If its busy and you work harder, you should relax when work is easier.

No.
But check how many hours they’re paying you for

Maybe, it’s a possibility they are testing you.

No, it's ok.