Why did you come back positive for cocaine when you have not did it?

some chemicals in cocaine are absorbed by the skin. so if you were to handle large amounts of cocaine it could result in a positive. also some of the very fine powders could float in the air and be breathed in and/or ingested. another consideration would be human tampering. could somebody have slipped it in your drink or food? maybe laced a cigarette or tobacco pipe? have you done any other drugs that could have been mixed with cocaine?