In one instance, a thief broke into a black Mazda between 11am and 11.45am on Sunday 16th October on a track known locally as Coffin Lane at Laughton Woods, near Gainsborough.
While the owner was walking in the woods an offender smashed a window and stole around £20 cash.
A similar theft happened in Susworth Road at Scotter, near Gainsborough.
Between 11am and 12pm on Sunday 16th October, a silver Mercedes was broken into and a Blackberry phone, a Nokia mobile and a number of CDs were stolen while the owner of a car was walking in nearby woods
All walkers are being urged not to leave any valuables in their cars when they are left unattended.
A Lincs Police spokesman said: “We would encourage all motorists to not leave anything of value on display in your vehicles. In more rural areas, thieves can operate away from view.”
Anyone with any information can call Lincolnshire Police on 0300 111 0300 or call Crimestoppers anonymously on 0800 555 111.