“It’s not that the absence of a sign somehow gives ICE some authority to enter. … The sign’s not necessary,” says Lucas Guttentag, a Stanford Law School professor and former Homeland Security official.

“An employer could, in theory, give consent to allow law enforcement to enter,” he says, but the posted notice makes clear that the employer does not consent. Mr. Alverson’s door sign, for instance, notes that ICE’s administrative warrants aren’t valid there.