ConceptMap has two search parameters of type reference that search uri properties, source-uri and target-uri:
http://hl7.org/fhir/STU3/conceptmap.html#search
Is that an error in the spec? One would expect parameters that search uri properties to be uri-type parameters. Conceptually it's not that difficult to imagine how a uri property could be searched as a reference, but the section on searching references makes no mention of uri's:
http://hl7.org/fhir/STU3/search.html#reference

The properties that these params search (sourceUri and targetUri) are "[x]" properties, and there are also params that search the corresponding reference properties (sourceReference and targetReference), so I wonder if this is a copy-paste error.

If this is not an error, then the section in the search spec on reference-type params should be amended to describe how uri properties are supposed to be searched as references.