new vecoffset;
public OnPluginStart()
{
  HookEvent("grenade_bounce", OnBounce);
  vecoffset = FindSendPropInfo("CPhysicsProp", "m_vecOrigin");
}
stock GetGrenadeIndex(client)
{
  new grenadeindex = -1;
  while((grenadeindex = FindEntityByClassname(grenadeindex, "flashbang_projectile")) != -1)
  {
   if(GetEntPropEnt(grenadeindex, Prop_Send, "m_hThrower") == client) return grenadeindex;
  }
  return 0;
}
public OnBounce(Handle:event, const String:name[], bool:dontBroadcast)  
{
  new client = GetClientOfUserId(GetEventInt(event, "userid"));
  new grent = GetGrenadeIndex(client), Float:location[3];
  GetEntDataVector(grent, vecoffset, location);
  for(new i = 1; i <= GetMaxClients(); i++)
  {
   if(IsClientInRangeOfPoint(i, 50.0, location) && i != client)
   {
    //флешка попала, твои действия
   }
  }
}
stock Float:GetDistance(Float:pos1[3], Float:pos2[3])
{
  return SquareRoot( Pow(pos2[0] - pos1[0], 2.0) + Pow(pos2[1] - pos1[1], 2.0) + Pow(pos2[2]-pos1[2], 2.0) );
}

stock IsClientInRangeOfPoint(client, Float:radius, Float:cpos[3])
{
  if(!IsClientInGame(client)) return false;
  new Float:ppos[3];
  GetClientAbsOrigin(client, ppos);
  if(GetDistance(ppos, cpos) <= radius) return true;
  return false;
}