My boss caught us, too.
He decided he was going to start doing end of shift check offs on the trucks. We were at the gas station filling up when he did the check off. He opened the driver side door and saw the sign....
My partner was able to save our hides by saying "Oh, we found it on the side of the road."
My boss was very skeptical but allowed it.
It was at this moment I fully realized that my boss is a good boss. Not a good manager. And puts too much effort into the wrong things. But a good boss.