Word about salaries gets around via the employee rumor mill. The new highly paid employee will tell their co-workers over drinks, or accidentally leave a pay stub on their desk, or someone in HR gossips with their friends. Trying to keep secrets never works when there are no real penalties to prevent it.

In the US at least, employees are explicitly allowed by federal labor law to discuss compensation with each other. Employers generally can't retaliate against them for doing so (although like all labor laws that one is occasionally broken).