Mr Hillis, who is from Wexford, is feared to have fallen foul of a drug trafficking gang,
Over the last few years, members of the negotiation unit (NNU) have assisted the local authorities in securing the safe return of Irish citizens in Nigeria, Ghana, Sudan, South Africa, Cambodia, Brazil, Algeria and Romania.
Gardai have been deployed on the ground dealing with incidents in Ghana, Nigeria, Sudan and Cambodia and worked closely there with officials from the Department of Foreign Affairs and local authorities.
Some of those who disappeared were victims of kidnapping by local crime gangs, seeking a ransom, while others were held for other reasons.
Garda negotiation specialists have helped secure the safe return of eight Irish citizens, who had been held against their will in foreign countries in the past three years.