Can an employer inadvertently recognise a trade union?

It is highly unlikely that an employer would inadvertently recognise a union. If it did the recognition would be voluntary and it could derecognise the union. In the absence of a written or oral recognition agreement, there would have to be unambiguous evidence that the employer had customarily negotiated with the union on at least one of the following: employees' terms and conditions of employment, engagement, non-engagement, termination or suspension of employment, allocation of work, disciplinary issues, membership or non-membership of the union, facilities for union officials, or machinery for negotiation and consultation.