Sincere question from a non-scientist who struggles with the idea of how to make use of existing data without accidentally P-hacking:

Is it still P-hacking if you stumble upon a correlation in the historical record (after stumbling around for a while), call it a hypothesis, and then stick with it long though to gather a statistically significant amount of _new_ data to support it?

More broadly, are there ways to "go on a fishing expedition" that are still scientifically valid?