Lincolnshire Police are giving local residents advice and seeking information on a spate of Lincoln burglaries at night.
In the last month, there were 20 offences overnight, mainly due to unlocked windows and doors.
According to police, offenders focus especially on ground-floor flats.
They usually get in by reaching though small windows to open larger ones to get in, or just walking through unlocked doors.
Offenders creep in while the occupants are sleeping, stealing items such as cash and cards.
DI Suzanne Davies said: “We are targeting patrols to tackle this issue and several lines of enquiry are being followed to bring potential offenders to justice, but prevention is key in terms crime reduction on this issue.
“Criminals are getting in through unlocked doors and windows. We would urge people to really think about their security at night – even when the weather is warmer.
If you leave ground-floor doors and windows open then you really are leaving your property vulnerable.
“It is also a good idea for families to talk to elderly relatives living in the community and explain these issues to them.
“Early reporting of suspicious activity is vital in terms of helping us to increase our chances of making arrests, so if you see anyone acting out of the ordinary in residential areas then please call us on 101.”
To give information on burglaries or handlers of stolen goods, call Lincolnshire Police on 101 or Crimestoppers anonymously on 0800 555 111.