Cps placed my grand baby with me pending drug tests. the tests are back. what does this mean?

my daughter and her 3 year old's drug tests are back. the cps lady called me and told me this and told me NOT to let my daughter have the grand baby under any circumstances. what does this mean? my daughter has a meth addiction and does other drugs. could the baby's drug test come back positive even though they would make her go to her room when they smoked drugs? she's claiming that they never did this in front of the child, but yet here we are. i don't understand.