"People have just sat on a table and they'll just come and grab your cellphone if it's laying on the table or you're holding on to your cellphone and you're not paying attention," San Francisco Police Capt. Garret Tom said.
The thieves are becoming so brazen, they're even snatching items from the restaurants themselves, like tip jars. The latest business hit was Tarrantino's restaurant.
The thieves travel in groups of three or four. Tom says most of those arrested are as young as 15.
"Recently, I guess they're out of school and there's nothing to do in the summer and they're just taking their chances," Tom said.
ABC7 News saw a whole lot of potential victims along the wharf -- diners placing their electronics on their table as they ate.
Ilista Penn from New Zealand left his cellphone on his table next to the street.
"We're in a tourist area like this; you'd like to think you're safe," Penn said.
Sacramento resident Diane Johnson did the same thing with her cellphone.
"We were visiting and talking and laughing and set it down just like that," she said.
Her friend Emmy Bantle is from a small Wisconsin town.
"If it happened in our town, we'd recognize them, so we wouldn't have to chase them, we'd just call their mom," Bantle said.
Police are advising restaurants to warn patrons if they see their electronic items on table tops.
Elizabeth Kennedy, who's is in charge of security at Lou's Fish Shack, says that's exactly what her staff is doing.
"They're very oblivious that something like this could happen so we try to advise," she said.
Police have made 13 arrests so far this month, but many of those arrested are juveniles, so they are back on the streets in little to no time. Police say they have done a good job educating residents about smartphone thefts, but they are working harder to educate tourists.
