Situation, worker violated a major safety policy. I was sent out to retrieve company vehicle etc. I was also told worker would be suspended pending the investigation. Arrived at work to find the "suspended" worker has returned. I believe my boss has unintentionally put me in a very awkward position as I am tasked with managing this group of workers. I also believe the message to the rest of the team has been compromised. What can I do?
Don't work above your role.
If you've followed instructions, just keep following instructions.
Contact your boss to confirm that the worker in question is authorized to be at work. If the boss says yes, then just continue on.
As to the "message" to the rest of the team: Your boss has (apparently) decided what that message is. You're a manager. You implement policy. You don't set it. The only way you could end up on the bad side of this is if you've decided to make some sort of individual "contribution" to the discipline that you don't have the authority to back up. Your best bet is to accept and implement your boss's decision.
Stay in your role.