Some of the bagels were poppy seed and one of my new-coworkers commented "well, guess we're not doing drug tests."
The "what can cause false positives" is a "this should be well known" and if someone has a positive result and claims that they've eaten poppy seeds, then get a hair sample and test that.
If you're in an industry that is likely to have random drug tests, it may be wise to avoid poppy seeds.
https://www.usada.org/spirit-of-sport/education/can-poppysee...