Police responded to the office at Mission Street between Eighth and Ninth streets just before 4 a.m. because a passerby called in a report of a broken window, police Sgt. Lyn Tomioka said.
People working for the campaign office saw the storefront on their way in around 8 a.m. and called police only to find out a report had already been filed, a campaign spokeswoman said.
"We feel like certainly it's a possibility that we were targeted," campaign spokeswoman Tiffany Burns said.
Burns said the only thing missing from the office was a Dell Inspiron 7120 laptop.
The office temporarily replaced the glass and did not close, she said.
Anti-war activist Sheehan is a congressional candidate in the state's eighth congressional district race against incumbent Congresswoman Nancy Pelosi, D-San Francisco.