He has been sent by the secret police to sniff out weed and then repot back to his police base regarding the illegal use of it.
When in the prescence of persons under the influence of this drug he becomes increasingly boisterous and sniffs them persistantly. Due to our intelligent tactics he does not have enough proof to persecute us as he has not discovered any actually marujana. It is thought that this boisterous ness is an attempt to knock a baggy or two out of someones pocket, thus successfully having enough proof for us to go down. And the persistant sniffing is expected to be him trying to work out where on our body these incriminating baggys are. But, we are ahead of him and do not keep drugs on our person.
Also, on entering my bedroom, he sniffed persistantly at my ash tray (which was at the time fixed to my coffee table with wax) again, trying to find any left over bits of bud or spliff ends still containing any. Also, he went for my bottle bong and attempted to escape with it. It is believed that any resin remaining in it would have been enough evidence had he been able to get away with my bottle bong that is.
A new theory has recently been developed that Charlie has been turned into a dog by some magic agent and the only way for him to escape this disguise is if he takes this magic agent evidence that we have been consuming illegal drugs. On the other hand it could be that this magic agent wants to get stoned. Hmmmm.