Once these underground cables are covered over with dirt again, a brick buried flush or slightly above the surface will mark their location.
I like to know exactly where buried lines and underground cables are — phone, water, propane, or septic field — either to avoid hitting them with a tiller or a post-hole digger, or in case I have to dig in that area in the future.
Rebar markers and the like can do the job OK, but you still have to mow and trim around them. My solution has been to bury one or more bricks facing flat and flush, or a bit “proud,” in the ground. Then it’s easy to just completely mow over my brick markers.