An interesting sideline developed from Jeanne B. Thorsen’s part-time job as a sales clerk at a clothing store: She became a private detective.

Mrs. Thorsen passed away Friday. The longtime Arlington Heights resident was 83.

Her road to becoming a private investigator started in a small boutique called Ruth Miller’s, located in the former Arlington Market shopping center in Arlington Heights.

“She was a sales clerk, but she used to catch people stealing the clothes,” says her son, Ray of Rolling Meadows. “They’d go in to try something on, and then try walking out wearing it.”

Her knack for catching thieves led Mrs. Thorsen to take a job in security at the former Wieboldt’s department store, at Randhurst Shopping Center in Mount Prospect, where she sniffed out shoplifters.

In 1970, Mrs. Thorsen decided to strike out on her own and open her own business. In doing so, she was one of the first women to be licensed by the state of Illinois, as a private investigator, family members say.

“She knew a lot of the Arlington Heights police members, who helped and supported her,” her son adds.

Outside the context of the clothing stores, Mrs. Thorsen’s clientele changed from shoplifters to married couples.

“I’d say about 90 percent of her business was following husbands and wives who were cheating on each other,” her son adds.

In order not to be observed, Mrs. Thorsen often resorted to wearing disguises, including different colored wigs.

“A lot of it was boring, just sitting in her car outside of hotels, waiting for someone to come out,” her son says. “But she enjoyed the night life.”

Her biggest adversaries were the angry husbands and wives who confronted her, once they had been discovered. For her protection, Mrs. Thorsen always took along her large German shepherd, Shane, who rode along in the back seat of her car.

Her role led to many newspaper and television interviews, during which Mrs. Thorsen always delivered the same advice, her son said: “You never know who’s watching.”

When Mrs. Thorsen wasn’t working, she was volunteering for the Wheeling Township Democrats, including serving as Midwest campaign manager for Samuel Shapiro, who briefly served as Illinois governor after Otto Kerner resigned to become a federal judge.

Mrs. Thorsen was preceded in death by her husband, Ray, and daughter, Barbara Liyeos. Besides her son, Mrs. Thorsen is survived by seven grandchildren and seven great-grandchildren.