a method comprising maintaining , by a device , communication with a monitoring unit ; determining , by the device , an absence of communication from the monitoring unit ; based at least on a result of said determining , starting monitoring of at least one variable related to distance moved by the device and / or time elapsed ; if the at least one variable meets at least one criterion , determining that the absence of communication from the monitoring unit is not due to a blind spot . a device securely attachable to a person may be configured to carry out this method , which may be user in offender monitoring .