The antonyms for the word "happens upon" are "misses" or "avoids." If someone misses an opportunity, they didn't happen upon it. Avoiding something means intentionally staying away from it, so if you avoid accidentally stumbling upon something, you won't be able to say that you happened upon it. Another antonym for "happens upon" might be "deliberately seeks out." When someone is searching for something, they aren't leaving it to chance to stumble upon it, they're putting in the effort to find it. So while "happens upon" implies something discovered by chance or luck, these antonyms suggest a more active role in finding or not finding something.