The car was discovered near Lurgan
The Honda car was discovered on the Annesborough Road near Lurgan on Wednesday night.
Dissident republicans were blamed by the police for a attack on Northern Ireland's main jail in County Antrim.
The 150lb home-made device partially exploded at the main gate.
The device was left in a white vehicle which pulled up outside the perimeter gate to the prison in County Antrim at about 2230 BST on Sunday.
Two prison officers were in buildings nearby, but no-one was hurt.
Chief superintendent Roy Toner said the police contacted the prison authorities as soon as they became aware of what he called "suspicious activity".
"We had no specific intelligence but we were on a heightened state of alert," he said.
"We are keeping a fairly open mind but our initial enquiries are focusing on dissident republican groups."
Meanwhile, Ulster Unionist MP Jeffrey Donaldson said demilitarisation should be postponed until "rogue paramilitaries" were dealt with.
The Lagan Valley MP said those responsible represented a "further threat to the peace process".
Abandoned
"It also demonstrates why the government needs to maintain a high level of security.
"Any thought they might have had for further reductions in security should now be set aside."
The dissident groups are opposed to the peace process in Northern Ireland.
They have been blamed for a series of bomb attacks in the province in recent weeks.
The police are looking for at least two men seen abandoning the vehicle containing the device.
Witnesses saw the white 4x4 and another vehicle approach the prison entrance and watched as the driver of the jeep got out and into the second vehicle, before it sped away.
The men ran from the Mitsbuishi vehicle, which had been stolen in Dublin earlier this month and fitted with false plates, to a waiting car, thought to have been the Honda found burned out.
Police have appealed for information on its movements.