Lookup Example: https://musicbrainz.org/ws/2/discid/TtGgcRAfA9mt3DfSB1UjyN4GU30-?fmt=json
Search Example: https://musicbrainz.org/ws/2/cdstub?query=discid:TtGgcRAfA9mt3DfSB1UjyN4GU30-&fmt=json

In XML, both result in <track-list count="N"> (with lookup providing contents and search an empty list with just the count).
In JSON, lookups give track-count, but search gives just count.