Radley Balko has a running list of all the materials that field tests have mistaken for drugs:
Sage, Chocolate chip cookies, Motor oil, Spearmint, Dr. Bronner’s Magic Soap, Tortilla dough, Deodorant, Billiards chalk, Patchouli, Flour, Eucalyptus, Breath mints, Loose-leaf tea, Jolly Ranchers