You might wish to give notice of inspection to check smoke and carbon dioxide detectors or change air filters and do so. Check around to see if anything stands out as to possible drug use. The following day send a notice to each tenant that the use of illegal drugs will not be tolerated and that any future signs of illegal drug use will result in termination of the lease. Don't say what you saw or did not see, let them guess that they screwed up and left something laying around that you saw. Check with your neighbors to see if they have notice anything and to call you if they do. Plan on getting new tenants when lease is up.