North Wales Police say they want to make all owners aware of recent incidents in which vans have been targeted and items stolen from inside.
In a warning issued through Neighbourhood Watch the police say: "It is believed the offenders are using an electronic device which can open the vans without causing damage.
"We would therefore urge owners to empty your vans of all contents overnight or when leaving the van unattended for a long period of time. Vans have been targeted in Abergele all the way through to Holywell.
"If you noticed anything or anyone suspicious please contact North Wales Police by dialling 101. Alternatively, you can provide information anonymously by calling Crimestoppers Wales on 0800 555 111. North Wales Police would like to urge residents to always report suspicious activity to them at the earliest opportunity."