What Drugs Can You Do and They Dont Show Up on a Rehab Drug Screen?

My step mom was supposed to be recovering from drug use. Now we think she is still doing them, she is in rehab and is passing the test but she is acting as if she is doing something we have found sand like crystals in the toliet. We don’t understand how in the world she is passing these test and we want to help her but if we can’t figure out what she is doing we will never know. Thanks for any help!!