Hi Hans51, Your question is an excellent one. There does seem to be a nuanced distinction between being caught and being seen. Based on my own intuition, I would say caught implies witnessing somebody doing something when they were not intending to be seen.
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Hans51The gang was caught unloading the cigarettes.This means while the gang was unloading something illegal, the police captured them in the act.
Hans51I caught him stealing the money.He was in the process of stealing money from your dresser and you happened to walk in on the act.