The theft of cash occurred at about 3:55 p.m. Dec. 22 at Lanphier Day Spa at 20 West Ave. in Darien, police said.
But the theft was reported on Dec. 26 by the management at the day spa, police said.
"An unknown subject" stole the cash while "the spa was extremely busy and the boutique room cash register had been left unattended momentarily," police said.
The incident is under investigation, police said.
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