Several Costa Mesa residents who typically park their cars on the street overnight got an unpleasant surprise when they walked out of their homes Saturday. Someone had made off with their cars’ side mirrors, authorities said.

Thirteen mirror thefts were reported citywide Saturday, adding markedly to the 11 reported between December and February, said Costa Mesa police spokeswoman Roxi Fyad.

The largest concentration of thefts on Saturday was on 22nd Street and Mesa Drive in Costa Mesa’s Eastside, police said.

Join the conversation on Facebook >>

The mirror casings were left intact and only the glass mirrors, valued at about $400 each, were stolen, police said. Most of the vehicles targeted were made by General Motors.

Police said they don’t know who is stealing the mirrors, but they suspect the motive is financial.

“We haven’t found out yet if they’re selling them,” Fyad said. “We can only assume that it has some sort of financial value.”

Authorities are encouraging residents to park their cars in a garage and be vigilant about anyone lingering around vehicles in their neighborhoods.

“We wanted to let our residents know so they can stay alert and be on the lookout for it,” Fyad said.

hannah.fry@latimes.com

Twitter: @HannahFryTCN