Shoppers who had their valuables on show were targeted by undercover officers in a crime prevention operation in Chatteris on Wednesday.
Plain-clothed officers spotted a number of people who had left themselves open as a potential target for pickpockets.
Postcards were dropped into bags to show how vulnerable their valuables were.
The operation was held following a spate of purse thefts in the town and involved officers and the community safety partnership.
The New Horizon bus was in High Street where crime prevention officer Julie Bailey handed out purse bells and safety bungees.
She said: “We identified four people who were at risk of having their purse stolen. When we spoke to them they were shocked to discover that a note had been dropped in their bag without them realising.
“We also noticed a number of drivers had left windows open after parking their cars and some vehicles were even unlocked.
“Our message is simple: If it’s left on show expect it to go.”