The discovery was made by a female store worker who noticed a small camera staring back at her as she tried on clothes in a fitting room on Saturday afternoon.
Jim Amormino of the Orange County Sheriff's Department says the camera was concealed with black electrical tape and was very small - only a half inch to two inches at the most.
Marsha Kmec of Mission Viejo says she always checks the fitting room before trying on clothes.
"Call it paranoia or call it being skeptical about being in a changing room and thinking there are other eyes on you, but it is something I think about often," Kmec said.
How long the camera was in place is not clear, and who could have placed it there is unknown.
Authorities now going through the video to see if there are other victims or possibly the suspect caught on tape.
"Investigators are reviewing mall security surveillance film as well as surveillance video from the Gap store," Amormino said.
In a statement, the Gap said, "The safety and security of our customers and employees is our utmost priority. We at Gap, Inc. were concerned by the discovery in our Mission Viejo Gap store and immediately notified law enforcement to ensure a prompt investigation. We conduct privacy assurance audits of our stores, and have intensified these audits in the surrounding stores. We believe this to be an unfortunate and isolated incident."
Anyone with information about the case is urged to call Orange County Crime Stoppers at (855) TIP-OCCS.