When offline cannot log-on – cannot find company domain

Several users have reported when working offline at home they can no longer log onto their laptops. They are receiving a message that the Company Domain cannot be found however if they use one of the firm admin accounts on the same pc they can log onto the domain and connect via vpn as themselves.
This just started happening within the last 3-4 weeks without any common bond between the reports other than they are laptops. Models vary as well as office/city/country.
If the user brings the laptop into the office and connects via the LAN line they can log-on and then things work fine after that.
We are running Windows XP SP2.