Foreign key without referenced columns specification
The behaviour is not explicitly mentioned in any of the official SQL Server documentation I am familiar with, but the 1992 Draft SQL Standard (section 11.8.2.b) does say:


  If the &lt;referenced table and columns&gt; does not specify a &lt;reference column list&gt;, then the table descriptor of the referenced table shall include a unique constraint that specifies PRIMARY KEY. Let referenced columns be the column or columns identified by the unique columns in that unique constraint and let referenced column be one such column. The &lt;referenced table and columns&gt; shall be considered to implicitly specify a &lt;reference column list&gt; that is identical to that &lt;unique column list&gt;.


Translated, this means an implicit foreign key does reference the primary key of the referenced table. As others have mentioned in comments to the question, it is probably best to be explicit about the relationship though.