A method includes determining, by a computing device of a dispersed storage network (DSN), a source name for a data object to be scanned for missing encoded data slices. The method further includes issuing list source requests to the set of storage units. When a list source response is not received from a storage unit of the set of storage units within a response timeframe, the method further includes identifying one or more encoded data slices stored on the storage unit as potentially missing encoded data slices; determining a next level missing encoded data slice determination approach for the storage unit based on one or more of: a number of potentially missing encoded data slices, a performance goal, a network loading level, a rebuilding loading level, a predetermination, and an entry of a system registry; and executing the next level missing encoded data slice determination approach.