Further to my earlier post on this topic, I have spotted an interesting item on the BBC News web-site. This gives more information about the signs to spot a cannabis factory, and is well worth reading, plus there is an informative video.
Generally though landlords need to do regular inspections, and if a tenant consistently refuses to allow access, this should alert you to the possibility of criminal activity. If you just ignore the signs, you may have problems claiming for any damage later, on your insurance.
Also, if you are a landlord, it might be an idea now, to check to see whether this sort of thing would be covered on your current policy.