How should I deal with a cable found underground on my property?
The images show a cable similar to a spool I have of &quot;cable-in-conduit&quot; which is used for cable television service. Typical cable television cable drops, from the service point (tap) to the house can be 6 mm diameter, but the c-in-c will be slightly larger. It's hard to tell from the image, but even if it's direct-bury drop cable, it's not dangerous.
Are you able to view a service point for local cable tv service in either direction?
Cable television drops are buried less than 30 cm deep in many cases and are easily cut by accident. Cable in conduit ostensibly protects the cable, but a sharp shovel and a determined gardener can certainly cause damage.
Consider to contact your local cable television provider with the same query you've posed to the power company.
Allowing for your edit, determining that it is a power cable of some sort, you can check for active current with an ordinary compass, the magnetic type, not the circle drawing type. According to one web site, for alternating current, the needle will vibrate at the frequency of the electricity, likely 50 Hz in your country. If it is direct current, the needle will point in one direction as you move it about the wire.
A more expensive method is to use a clamp-on ammeter, if one is available. That is sometimes inconclusive as it often requires to separate the conductors to ensure an accurate reading.