This happened quite a bit during the MyRedbook days from the late 90s

[theking]

..to the first decade of this century..two call system with a spotter:

Couple trafficked women at hidden brothel in luxury apartments, California police say

Six women were rescued from several California brothels, including one hidden in a high-end apartment building, after a two-year investigation, police say.

Investigators also seized around $2 million and arrested a married couple accused of running the sex-trafficking operation in the San Francisco Bay Area, Milpitas police said in a statement.

David Davies, 57, and Larong Hu, 38, face charges including pimping and pandering and felony conspiracy, police said. The Santa Clara County District Attorney’s Office also took part in the investigation.

Upscale apartments in a Milpitas complex with rents up to $5,000 a month housed one of the brothels, unbeknownst to neighbors, KNTV reported. The couple is accused of advertising online and forcing women from overseas to work in the brothels by seizing their passports.