The smugglers did not realise that their conversation was being recorded.

The smugglers did not realise that someone was recording their conversation

Someone did not realise that the smugglers were recording their conversation

Conversation was recorded when the smugglers did not realise

The smugglers recorded their conversation without realising

All of the above

The sentence is in Past Continuous Tense.

Present Continuous Tense

Active Voice: Subject + is/am/are + First of verb + ing + object.

Passive Voice: Object + is being/am being/ are being + third form of verb + by + subject.