Is it permissible to recite the supplication for expiation of a sitting immediately after leaving a gathering, with the intention of concealing the act from people?

The original principle regarding the supplication for an assembly is that it should be recited at its conclusion, when one intends to leave. So, whoever sits in an assembly where there is much idle talk, and before rising from it says: "Glory be to You, O Allah, and with Your praise, I bear witness that there is no god but You, I seek Your forgiveness and repent to You," then whatever occurred in that assembly will be forgiven for him.

Its place is before rising from the assembly and upon its conclusion. It can be recited silently for fear of ostentation, unless there is an overriding benefit in expressing it openly, such as if the person is someone whose example is followed, for the purpose of reviving a Sunnah. For concealing good deeds is desirable in Islamic law, unless there is an overriding benefit that calls for their open expression, because it is more conducive to sincerity and further removed from self-admiration and ostentation.