Cameras disguised as smoke detectors are routinely sold by Alibaba and Chinese websites for as little as $20. advertised as a "security item"
The woman who found the camera in her rental apartment lived in Sacramento, California and has moved out. the real disturbing part is that she has no idea who installed or how long it had been there. the good news it that it recorded to an SD card, not streaming to a recording device. More disturbing, if the card was there for a long time; someone was routinely accessing her apartment and replacing the SD card for a new one.
Another one, a couple from Indiana visiting Longboat Key, Florida, found a camera hidden in the smoke detector in the master bedroom of their Airbnb, the owner of the Airbnb was arrested and charged with voyeurism as he admitted he had installed it to spy in sexual activity in the bedroom.
At the spa, I think if you are spied on, turn off the lights. Most cameras can't record properly in the dark; infrared and pinhole cameras cost thousands. I doubt you local spa is spending that kind of money just to see your fat ass. LOL