قالوا وَأَقبَلوا عَلَيهِم ماذا تَفقِدونَ

They said, as they turned towards them: ‘What are you missing?’

EXPOSITION

As they turned towards them: this is added here to further illustrate the point we made in the last verse, that they were already on their way out when the guards and servants raised the alarm.[1] It is likely that they were travelling in a caravan out of the city and the verb aqbalū (turned) is used to illustrate how they began returning to the city to be taken to Prophet Joseph (a). This latter understanding is reinforced by the later verse, Ask [the people of] the town we were in, and the caravan with which we came (verse 82), which suggests that the caravan with which they were travelling were witnesses to the events that transpired.

What are you missing?: while this is not a denial of stealing per se, it is a natural reaction of someone who is surprised and taken off guard by such a sudden accusation.

[1] See also Mizan, 11/223.