You cannot give it back as technically you could be charged with supply! I would make sure my crew mate saw it and was aware of the situation as if you just closed your hand over the bag there is always the chance of someone being very vigilant and seeing you try to hide it. They then could assume that you were hiding it with the intention of keeping it. I would not wait until the end of the shift to give it to the police! What happens if it falls from your pocket or someone accidently finds it on you during the rest of your shift? I would get the Police involved straight away. They have made the decision to use illegal drugs and should accept the consequences of that decision.