Patrols have now been stepped up at heritage sites after police and a field officer found two holes dug out and the turf replaced inside the ground of Dinas Bran Castle in Llangollen.
CADW, the Welsh government body in charge of designated ancient monuments, received reports in early February of evidence of metal detector use and digging within the grounds of the castle.
A site visit was made by field officer who located evidence of digging. CADW are unable to identify if any artefacts of value have been removed from the site.
PC Allen from the Rural Crime Team said: “To use a metal detector within or upon a scheduled ancient monument such a Dinas Bran without the consent of CADW is an offence and the matter is being jointly investigated.
“Patrols have been stepped up at our heritage sites in North Wales and I urge any members of the public who see metal detecting upon our ancient monuments to get in touch with CADW or the police."