I'm pretty sure in MN employers are required to give 24 hr notice. We have tons of these doctor offices called occupational health clinics, and it's literally just medical staff that preforms drug screening, physicals, and job related injuries. Very easy to smuggle in urine. Show up for your appointment, and viola.

Now large federal or state jobs can have on site random screenings. My uncle was a union sparky in Alaska, he said he would just abstain while on those big jobs but the pay was worth it.