People back feed panels all the time(cause they are stupid & idiots)! Turn off 200 amp breaker check dead/no voltage, unbolt phases & neutral. Wire exposed ends of both phases and neutral together to make a direct short (use a bare 12 or 10 gauge skinned length of wire)Bind it tight! Tape it up.That way if they try to back feed the panel in the future it will trip/short out their feed or generator. Otherwise if they back feed later on and you only taped up the ends separately you will have energized cables in your hands and assume they are de energized.Seen it happen all too often. Be safe,people are stupid. More people die of house voltage 120/240 than high voltage 277/480-138k

Also remember the first thing to do when checking for voltage is to check the voltage meter to make sure it works.