Yes, you will still have to pay the fee.
Actually It normally only takes a few days to find out something for an adultery case. There are usually two possible reasons if the third person is not found after the process. First, the third person has left Hong Kong/ the relevant place or nation ,or the third person does not permanently live in Hong Kong/ the relevant place or nation. Second, your partner is not having an affair.
We will usually advise you to wait for a few days, or even a few weeks for another part of surveillance to be carried out.