When trying to reference a Custom Object(s) in a Smart List with a Filter, there are no filters for the Custom Object(s).
Yet, when checking within a Smart Campaign, there are Triggers for the Custom Object(s).
Filters are not included in the original design of the Marketo Application for Custom Object(s) setups that act as an intermediary/bridge between two other objects.
Check the object setup within Marketo Admin > Marketo Custom Objects; how many linked objects are there? If there are two linked objects that means the selected Custom Object is an intermediary/bridge between the two linked objects. For these, there will not be any Filters, however there will be Triggers for these objects.
Who This Solution Applies To
Clients with Marketo Custom Objects