If you did in fact owe the cable company money (or equipment) at the end of your service, then you will have to pursue upper levels of their organization to get someone to acknowledge (find) the debt and accept payment, hopefully in agreement for deletion once it's paid. If this is the case, it is possible that they charged it off or otherwise sold it off, and the agents you reach on the telephone cannot see it (it may not even exist in the computer records for the original creditor anymore). That the collection agency can't find it is more troubling - if they have no record of it, then they have no collection authority (can't have authority to collect a debt that doesn't exist) and so have no reporting rights.
If you had service with them but can show that in fact you did not owe them any money at the end (including any equipment you needed to return), then dispute the debt as "not mine" and have the CRAs get them to verify it (or remove it, which is more likely in this case). In this scenario, it has to be a reporting error - the cable company is reporting it on your credit, but it belongs to someone else.
If it's appearing on your reports, then it is being reported by someone. Either the original creditor (the cable company) or, more likely, the collector who currently owns the account, is reporting it. If they truly cannot locate the account/debt when you contact them, then they must be reporting improperly - someone else's debt is being reported on your credit - and that means they have to fix it, especially once they receive notice of your dispute, because failure to do so leaves them liable for FCRA violations. But again, this is only if there is no debt that belongs to you (in the case of the original creditor's reporting). If the collection agency has no file/account to collect on, they have no right to report, period - regardless of whether you do owe the original creditor or not.
