For a long time I didn’t understand why he kept leaving the office for an hour or two several times a week, each time with a different excuse. But when he announced that he was leaving the company, the penny dropped (der Groschen ist gefallen) – he had been going to job interviews!
The origin of this idiom, and presumably the very similar German equivalent, refers back to the mechanism of penny-in-the-slot machines (which were used to sell sweets or cigarettes for example). When the penny dropped, the machine worked.