No matter which method we use, we will never find everything we want to find. The larger the amount of data, the greater the risk of overlooking something. This was also true for Luhmann. Who knows what else he would have written if he had found all his Zettel. But the opposite is also possible: Perhaps he would not have written some things if certain Zettel that he did not find had refuted a thesis. And even if he/we find all Zettel, we certainly won’t find those that we didn’t write. Something is always missing. We just don’t know what.
All Science (every piece of work) is also a product of coincidence. You recognize what you recognize. Later, someone else recognizes something else.