It is a question inspired by the one about handing the bribe: What should a project manager do if he/she is asked to pay a bribe?
Mine is a bit different. You're a project team member in the middle of a project and you accidentally overhear salesperson talking with the client in the way which clearly indicates there was bribery involved to get the project. You personally wouldn't hand the bribe out even if directly asked by your superior. Anyway, no one asked you about that and no one got you involved in that. You didn't get your hands dirty - you only learned about the situation. What should you do?