'onto someone' means to be close to/or having discovered the truth about an illegal or undesirable activity that someone is engaging in.
'undesirable' covers a multitude. If someone is trying to cause trouble for someone, they may confront the person with, "I'm onto you, feller-me-lad. I know your little game. Keep it up and you'll be sorry."
'be onto something' meaning to have an idea or information that is likely to lead to an important discovery. Often used in movies about the reporter sniffing out a story for a big scoop, and he has a hot lead - 'he's onto something' that could lead to the exposť of a big shot.
'come up with' means to produce something, especially when pressured by time, or challenged; 'to deliver the goods'. Everyday, a political cartoonist has to read the news to appear in the next day's paper, and come up with a satirical cartoon comment on the news before the paper goes to bed.
'goes to bed' = is printed.
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