SAN FRANCISCO » San Francisco police officers helped Apple Inc. investigators look for a missing iPhone prototype that was left in a city restaurant in July, the police chief said, the second time in two years the company has lost an unreleased smartphone.

Police Chief Greg Suhr told the San Francisco Chronicle that four plainclothes officers accompanied two Apple investigators who searched a San Francisco home for the iPhone prototype.

Apple employees who contacted the department asking for help finding a lost item conducted the house search after asking the resident’s permission, and the officers did not enter the home, according to police.

Apple tracked the smartphone to the home using GPS technology, but the gadget wasn’t found there, said Lt. Troy Dangerfield.

Apple officials have declined to comment on the case.

The Cupertino, California-based company is reportedly planning to release a new version of its popular iPhone this fall.

The SF Weekly newspaper reported that Sergio Calderon, who lives in the home, said he believed all six people were police officers and would not have let the two investigators inside if he knew they worked for Apple.

Suhr said he didn’t know how the Apple employees presented themselves to Calderon.

"The reason we do civil standby is to make sure there isn’t a problem," Suhr told the Chronicle. "Whatever conversations the employees had with the resident, I can’t say."

Suhr said it’s not uncommon for San Francisco police officers to help private investigators.

There was no record of the officers’ involvement in the search because Apple did not want a lost property report created, which was within the company’s right, Suhr said.

Last week, two men were charged with misdemeanors for selling a lost Apple iPhone 4 they found in March 2010 in a Redwood City, California, bar. That phone was sold to the gadget blog Gizmodo.com for $5,000, prosecutors said.