Pluperfect / Past Perfect progressive subjunctive
There is a progressive form of the pluperfect subjunctive tense called El Pluscuamperfecto progresivo de subjuntivo. It often expresses actions that hypothetically might have [not] been taking place in the past.
Yo no creo que ellos hubieran/hubiesen estado trabajando en esas circunstancias.
I don't think they would have been working under those circumstances.