Hope fortune be followed by the unlucky thing happened to you today and police can catch the thief. By the way, the logic applied by the police doesn't make sense. They can also investigate illegal use of your Octopus card, Bank withdrawal card......etc (assuming you have reported loss with Bank (s) and Octopus Company).
Every Octopus card has a number. - If you have applied auto-value added with bank, you may check the number with bank. - If you purchase by Octopus, the receipt should have record of your Octopus number. Receipt of adding value also shows the number.