On the Facebook Ad Account schema (here: https://fivetran.com/docs/applications/facebook-ad-account) we have a table called "custom_audience_history", which most probably uses the Custom Audience endpoint on the Facebook API (https://developers.facebook.com/docs/marketing-api/reference/custom-audience/v9.0).

This endpoint only returns the original Ad Account the audience was created in. So if an audience X is inside 3 accounts, this endpoint will only return the original Ad Account.

Facebook has an endpoint called Custom Audience Shared Account Info (https://developers.facebook.com/docs/marketing-api/reference/custom-audience/shared_account_info/). When you hit this endpoint ({custom_audience_id}/shared_account_info), it returns an object with all the accounts the audience is shared with.

Adding this endpoint would allow monitoring of audience sharing, in order to comply with GDPR related regulations and internal privacy controls.