A bag of rubber bullets was found by members of the public after Cumbria Police "misplaced" them.
The baton rounds were found in a "bum bag" style package on a grass island in Kendal.
Two workers had come across the pouch when doing some general maintenance on signs outside their offices.
Without opening it they assumed it was someone's personal belongings and left it on the wall in case the owner returned.
When no one had collected it by Tuesday, they handed it in to their line manager.
Dave Armstrong, managing director of Cox and Allen Building Contractors, knew there was something wrong straight away.
He said: "I rang the police to alert them because I didn't want this in the public area.
"If someone was to leave some shotgun cartridges lying around they would lose their licence.
"I think it's worrying that things do go amiss. I would've thought they would be counted out and back in again."
The bullets came in five tubes around half an inch in diameter and four or five inches long.
They are usually used for riot control and to disperse protests, but Cumbria Police has not confirmed why they were being used.
The College of Policing refers to them as a "less lethal option".
Inspector Paul Telford, of Cumbria Police, said: "Our strict issuing procedures revealed the rounds to be missing and we made immediate efforts to find them.
"I'm grateful to the finder for rightly and responsibly handing them to police.
"I'm confident no-one has been put at risk as a result, and I have reviewed the measures in place to minimise the possibility of this happening again."