I do drug testing in my job. We have to watch the urine leave the body. We do it randomly...sometimes several days in a row, sometimes not for 3 weeks. We contract with a federally recognized lab. People still try to do things to fool the tests and there are still false positives occasionally that have to be sent back to the lab for verification. We also get false negatives - people tell us they've used but it doesn't show up on the test.
Drug testing is a useful tool but it's not as straightforward as it seems. I wouldn't want to depend on it as a way to establish trust in my home.