Many a time, HR becomes a thankless job in situations of conflict of interests because of the work culture of an organization and the total quality of its management. It is said that the Moghul Emperor Aurangazeb would have three commanders for every expedition without each of them knowing the fact. There are many Aurangazeb-like CEOs in modern business enterprises. They have their own informal espionage system within the organization. Most of the moles eventually become time-servers to settle down their personal scores with their co-employees or pass on some false information of their choice about managers or others employed in position of confidence just to please the CEO or maintain their proximity to power centres within the organization.
Coming to the situation described by the poster and the questions raised, most probably the bonus mentioned is performance reward. Disparity in performance reward drives some employees to relinquish their jobs means, your performance appraisal system is biased. Similarly, once the amounts got disbursed, individual shares can not be a matter of secret any more. Your accompaniment to some people on the dining table might have been quite frequent and as such more visible. It might be quite trivial and sheer coincidence. But the mole's crooked mind would have distorted vision only and he/she might have exploited the situation to his/her advantage. In such awkward situations of deceit and despair, you should have the native intelligence to convincingly prove your innocence. Try not to be totally defensive or emotional since your credibility is at stake. If such fair and ideal approach fails to work, even no harm in making the confusion of the impossible type of person worse confounded for the sake of your survival.