Cabling will not necessarily show on building plans. It certainly does not appear on our building plans (built 8 years ago). And services to lots can most certainly run through other lot’s internal walls.
The OC is responsible for the repair and maintenance of services if they service more than 1 lot (common services). But you did get permission from the OC to demolish the wall. The OC permission should have stated that you would be responsible for costs of restoring any services to other lots that you might accidentally disrupt.