I was looking at spy gear, and found these drug tests. They can be used to test surfaces for drug residue, I found one for Cocaine, and one for Weed, I'm not sure if there are any others, though, but I think these could be useful for an investigating RLSH to have evidence to give to police if they a) Suspect someone is using drugs, or b) think that there is a drug lab or grower nearby, or c) think someone could be selling or using in a particular area.
http://www.theprotectionpros.com/home/tpp/page_229_34/cocaine_drug_test_kit__dnc.html
http://www.theprotectionpros.com/home/tpp/page_228