This is why companies that have "salary secrecy" policies may be treading on thin ice. What I mean by this is that if you have a policy that forbids employees from discussing their wages may be violating the NLRA. You can forbid an employee from revealing to or discussing wages with someone outside the company, but it is shakey at best to forbid employees from discussing it internally. Terminating such an employee may result in a charge of an "unfair labor practice" (ULP) which will be rectified by reinstatement of the employee and payment of back wages.
Concerted activity is not just restricted to wages, however. According to the website Law Memo, "Protected concerted activity is that activity engaged in for employees’ “mutual aid or protection.” Such activity includes employee efforts to improve working conditions and terms of employment. If an employee is engaged in protected concerted activity, an employer may violate the NLRA if, in addition:
- The employer knew of the concerted nature of the employee’s activity;
- The concerted activity was protected by the Act; and
- The adverse employment action at issue (e.g., discharge) was motivated by the employee’s protected concerted activity.