Is the percentage taken by salespeople and purchasing agents from some suppliers, without the employer's knowledge, permissible (halal) or forbidden (haram)? And if it is forbidden, does silence about it constitute a sin for whoever knows about it?

The commission that an employee takes from the supplier is not permissible for him unless the employer knows and permits it. You must explain the ruling on this to whoever does it, advise him, and enjoin him to good. If he does not respond, warn him that you will inform the employer. Then, inform him if he persists, unless you fear harm to yourself.