As long as the award is mentioned in the "books" (and it is because it was credited to your account which in my mind implies a bank transaction or something of this sort which I believe was sent from a company bank account) it is ethical.
Can this award be secret? YES. Maybe management want's to avoid dissatisfaction among your peers and in this case this is a legitimate decision.
Why did you received this award?
Well, this answer should be given to you as part of the appraisal process. No manager will give someone, something without explaining why, for the obvious reason that rewarded performance gets to be repeated which is good for the company in terms of work performance.
What should you do?
Continue doing a good job. There is nothing unethical or of criminal nature (if the bonus of course is not a bribe or meant to influence your decisions in favor of criminal or illegal matters).