Jose Ignacio de Juana Chaos is believed to have absconded despite being due to appeal an order for his extradition.
His barrister, who confirmed he has not heard from the 54-year-old since March, argued for the challenge to be stayed indefinitely rather than dismissed.
Sean Devine said it was accepted that de Juana suffers from mental health problems.
The lawyer said: “He spent 18 years in isolation in custody in Spain. The evidence in the court below is if he were to return there it could lead to his death.
“In those circumstances it may be the case that he is simply in hiding and not facing up to the rigours of this appeal and obviously wants to avoid an adverse result.”
De Juana was jailed for his part in an ETA campaign of 25 murders.
Spanish authorities were granted an order for his extradition in March to face a new charge of glorifying terrorism.
The Recorder for Belfast rejected claims that he would not receive a fair trial and may face harsher punishment for his political beliefs.
But de Juana, who had been bailed to live with his wife in west Belfast while fighting the extradition proceedings, has not been seen for more than five months.
Mr Devine confirmed on Friday: “He hasn’t surfaced. We haven’t heard anything from him.”
The Basque is wanted over the contents of a letter read out at a rally in San Sebastian the day after his release in August 2008.
The message was allegedly given in his name, with the charge against him heavily dependent on the Basque phrase which translates as ‘kick the ball forward’.
He faces the possibility of further imprisonment if convicted of the public justification of terrorist actions which caused humiliation and intensified the grief of victims and their relatives.
Stephen Ritchie, appearing for the Spanish authorities, requested that the appeal should be dismissed in de Juana’s continued absence.
He said: “This gentleman came to Northern Ireland a short time ago for whatever reason.
“He was sought by Spain, ordered to be extradited and has left the jurisdiction it would seem.”
Lord Chief Justice Sir Declan Morgan, sitting with Lord Justice Higgins, reserved judgment on how to resolve the case.