How does a flight or squadron communicate when radio silence is in effect?
USAF Instruction 11-205 Aircraft Cockpit and Formation Flying Flight Signals details the guidance and procedures for Air Force formation flight. It also indicates distress signals that have been standardized for use with North Atlantic Treaty Organization (NATO) forces. 

To get attentions, usually the wings are rocked:


  Attention in the air Execute rapid shallow rocking of wings.
  
  Loosen Formation Fishtail the aircraft.


The following are some of the distress reasons and corresponding signals:


  Bailing Out or Ejection One or both clenched fists pulled downward
  across the face to simulate pulling an ejection face curtain.
  
  Intercepting Signals The  intercepting  aircraft  positions  itself  in
  front of and usually to the left of the intercepted aircraft and rocks its wings.  This is a signal that the interceptor wishes the other aircraft to follow it.  A responding irregular flashing of all available lights in this case indicates distress


In case of unexpected danger/target or where time is critical, the flight is allowed to break radio silence. The instructions states:


  These signals reduce radio communications and permit pilots to communicate with each other during periods of radio silence.  Use the radios if the situation permits, when time is critical, or the visual signals are creating confusion. (emphasis mine)