The language in our rider says if you show up and are put to work you are guaranteed 8 hrs. Nothing about asking for extra work, or checking in. In the OP's case, if he were in my area, he should get his 8 hrs and a write up for failing to work as directed. Some would say he should be fired for job abandonment, but they don't know what that really means.
Even if you punched out on the board, if you expect 8 hrs of pay, you are still on the clock until you hit 8 hrs, so you follow instructions. If they release you, then you are clear.