When I last voted in a national election (in Germany, in case it matters) I observed that the election workers (the people who checked my documents, gave me my ballot paper, etc) would keep the slot of the ballot box covered with a sheet of paper until a voter was ready to cast their ballot. Then they would briefly remove the paper, let the voter put their ballot in the ballot box, and then cover the ballot box up again.
Why are they covering up the slot on the ballot box? Presumably it's to ensure that only completed ballots are put in the box - but I can't imagine what else they could fear could end up in the ballot box, it's not like the polling station was swarmed by an angry mob demanding to stuff their telephone bills into the ballot box?
I've seen this happen a number of times on TV, in various places around the world (usually as part of news coverage of an election). Sometimes the election worker uses paper to cover up the slot, other times they push a button on the ballot box to physically open the slot.