With insider trading, do I need to be caught in the act to be investigated or prosecuted?
No. Unless someone who is part of a scheme confesses, as in the movie Wall Street, direct proof rarely emerges. For them to investigate, prosecute, and convict you, the SEC and Justice Department do not need to catch you in the act of using the important, nonpublic information to make a trade or tip others. They use all the material they collect, including big data analyzed by sophisticated tools, to build a case on strong circumstantial evidence that points to the conclusion that you must have known the information and used it in your trade or tip.
Even though prosecutors nowadays prefer direct evidence, e.g. from wiretaps, to pursue convictions against insider-trading suspects, the guilty verdict against a former Goldman Sachs director in 2012 demonstrates that circumstantial evidence alone can still be enough to secure a conviction.