In the original context of HTTP being a way to distribute interlinked documents, a web application backend sending a 404 for an invalid database identifier is equally as valid as sending a 404 for an unrouteable path. The HTTP spec does not support a distinction between the two cases.

If you absolutely needed separate status codes for the two cases, you probably could use 410 Gone or 501 Not Implemented for unrouteable paths and 404 Not Found for bad IDs.