It seems that missing or unexpected unique constraints are not found either by the diff or diffChangeLog command.
I tried the same diff (exact same DB and config) using Liquibase 2.0.5 and it worked fine. Never encountered that problem before either.
I tried to check if it was because of a new column used in a new unique constraint or simply a new unique constraint using existing columns. It did not affect the result.
Note that corresponding new and unexpected indexes are found normally
Windows 7, Postgres SQL9.2