Since New Year's Eve, 10 grates have been removed from the curbs of public parking lots.
A total of 30 have been stolen since July.
The grates weigh hundreds of pounds each.
"Thankfully, we've had no reports of injuries or property damage, but missing stormwater grates pose a threat to pedestrians, cyclists and drivers alike," said Lt. Steve Fonke of Cary's Criminal Investigations Unit. "We encourage citizens to be on the lookout for suspicious activity and to call (919) 469-4012 to report a missing grate."
There are no suspects at this time.