It depends on a couple of different factors. First consider the drug and the volume of drug there is. A small amount of water based probably wouldn't be an issue because it would take an appreciable amount to be detected once its dried. Oil based is a different story because the oil won't dry quickly. Food grade Oils (ie olive, grapeseed, etc) all have a distinct smell that can be easily detected by scent trained dogs.
You also have to be concerned about cross over odors. Dogs are trained to detect a certian oder, they are not trained to detect a specific substance. If there is something that smells close---ie olive oil as in FOOD, there is a risk that the dog may show subtle interest what they are smelling. (Yes, an ideally trained dog wouldnt' but lets be realistic, there are levels of drug dog just like there are levels of police officer).
Dogs are not fullproof, but their noses are one hell of a lot better than humans. They also can and do give false positive reads. If you are acting jumpy and the dog gives the slightest hint of a reaction, that may be enough to give the handler suspicion not because of the dog smelling something, but because of how you are acting. At that point, it doesn't matter. You react in any way they suspect and they are going to go after you.
