What is this upside-down silver key icon I inherited a SQL Server 2005 3rd party server and database to look into to be able to generate reports and dashboards on.
On one table, I found a new icon: a blue-silver upside-down key, alongside my other primary and foreign keys.
When I right-click it, the context menu gives me the option to &quot;Script Constraint as&quot;, but when I try to &quot;Modify&quot; it, it opens the column design form (like primary keys), and yet, it appears in the &quot;Indexes&quot; section, not constraints.
Someone knows what it is called and what it is used for?
It's signifying a unique constraint. In SQL Server, a unique constraint is actually implemented as an index under the covers; this is why it appears under the Indexes node as well (I can't give you a good explanation of why it is not at least listed under the Constraints node, nor why they chose an upside down key instead of just a different color). 

You should see a result here to validate that this is actually implemented as an index:

SELECT index_id, type_desc, is_unique_constraint
  FROM sys.indexes
  WHERE name = N'IX_tblCustIndex';


You said that you saw "Script Constraint As..." - if you followed through with that it would have provided you a script something like:

ALTER TABLE [dbo].[tblCustIndex] ADD UNIQUE NONCLUSTERED 
(
  [some_column] ASC
) WITH (...a bunch of default options...)


But don't run it because this syntax does not use a name and it will just create a second, redundant unique constraint...

Here is a very simple repro, and proof that I get the upside-down key even with very basic syntax, on 2014:

CREATE TABLE dbo.foo(id INT UNIQUE);



   click to enlarge


Now, you can get a different icon if you create a unique index explicitly (and the only differences you'll see in sys.indexes are the name and the value of is_unique_constraint):


   click to enlarge


For other differences see this Stack Overflow answer.