Thieves used car jamming devices to prevent owners from locking their cars and stole items from three vehicles in one part of Sheffield over the last few days.
Police believe thieves could have used the transmitters, which interrupt signals from keys fobs, and prevents owners from locking their cars after there were no signs of damage on any of the vehicles.
The concerns come following a number of incidents, which Sheffield West neighbourhood police team dealt with.
Police said a driver parked their vehicle on Ecclesall Road at around 2.45pm on Friday and locked the car.
When the owner returned, items have been taken from the car but there were no signs of forced entry.
A similar incident took place on Albert Road, Gleadless, at some point between 11pm on Thursday, May 10 and 9.30am on Friday, May 11 where items were taken from a vehicle but there were no signs of damage.
A third vehicle was targeted on Backmoor Road, Norton, at around 2pm on Friday.
A spokesman said: "There has been a spike of thefts from vehicles which show no signs of entry, where the driver clearly recalls locking their doors and confident they can be attributed to thieves blocking their signals remotely.
"Drivers are pressing their key fob and walking away, but they are having their frequency blocked by thieves. The only way to know your car is locked is to check it manually, if your indicator lights flash, wing mirror folds in, or the horn beeps when you lock it, this is a sure sign your vehicle has locked, but the fail safe method is to give your door handle a little tug before you walk away.
"Basic jamming devices can have a range of 75 metres and effect a whole car park at the time of activation. Please try and remember to take this advice on board when leaving your vehicle."
Anyone with any information about the incidents should call police on 101 or Crimestoppers, anonymously, on 0800 555111.