It truly depends how you have designed your process and what your business team is looking to achieve overall.
If you have explicit maps for the various dimension then you can simply add a catch all map (e.g *,UNMAPPED or *,IGNORE) to each of the dimensions.
In FDM Classic world, what you're trying to do would be to set up suspense accounts to automap unmapped members. For instance, unmapped entities can go to SUSPENSE_ENTITY and unmapped accounts can go to SUSPENSE_ACCOUNT.
Like USER1211 mentioned, the easiest way to accomplish this is with a Like mapping on each dimension. You can have a mix of Explicit and Like mappings as long as the last map to get hit is the suspense mapping. Like maps are executed alphabetically based on rule name. To ensure the suspense map gets hit last, simply give it a rule name that will get hit last, e.g. Z_UNMAPPED_ENTITY.