The tax department doesn't have access to your banking records as a matter of course. They would need to instigate a formal legal procedure to get that, which they would only do if they had cause to be suspicious.
People who have investments overseas move large sums of money in and out of HK all the time - for example, far more money has entered my HSBC account here than I have earned while I have been here. So that in itself isn't an issue.
As to whether it should be declared to IRD, I believe the answer is yes. You are running a business which provided a professional service for which you were paid. Strictly I believe you need:
a) to register your business (as a sole proprietorship probably)
b) produce accounts
c) pay profits tax
I believe there is an exemption for casual earnings under HK$25,000 per year, but it sounds like you will be over that.
For ImmD if this service was simply an introduction rather than "employment" then I don't think there would be any issue for them.
In practice, of course, many many people in Hong Kong have casual earnings which are not declared to the tax man.