Nonclustered index storage on clustered columnstore
The "bookmark" is the columnstore index original locator (per "Pro SQL Server Internals" by Dmitri Korotkevitch). This is an 8-byte value, with the columnstore index's row_group_id in the first 4-bytes and an offset in the second 4-bytes. 
If you use DBCC PAGE to look at the non-clustered index, the 8-byte columnstore index original locator appears in the "uniquifier" column of the DBCC PAGE output. This shows that a unique non-clustered index does not need to include the columnstore row locator, whereas a non-unique non-clustered index does. 


The following code creates a columnstore-organized table with a unique and non-unique b-tree nonclustered index on the same column:

CREATE TABLE dbo.Heapish
(
    c1 bigint NOT NULL,
    c2 bigint NOT NULL,
    INDEX CCI_dbo_Heapish CLUSTERED COLUMNSTORE
);
GO
INSERT dbo.Heapish WITH (TABLOCKX)
    (c1, c2)
SELECT TOP (1024 * 1024 * 8)
    c1 = ROW_NUMBER() OVER
        (ORDER BY C1.[object_id], C1.column_id),
    c2 = ROW_NUMBER() OVER
        (ORDER BY C1.[object_id], C1.column_id)
FROM master.sys.columns AS C1
CROSS JOIN master.sys.columns AS C2
ORDER BY
    c1
OPTION (MAXDOP 1);
GO
CREATE UNIQUE NONCLUSTERED INDEX UNIQUE_c2 ON dbo.Heapish (c2) WITH (MAXDOP = 1);
CREATE NONCLUSTERED INDEX NONUNIQUE_c2 ON dbo.Heapish (c2) WITH (MAXDOP = 1);


We can see the size of the index row at different levels of the b-tree using sys.dm_db_index_physical_stats:

SELECT
    DDIPS.index_level,
    DDIPS.page_count,
    DDIPS.record_count,
    DDIPS.min_record_size_in_bytes,
    DDIPS.max_record_size_in_bytes
FROM sys.dm_db_index_physical_stats
(
    DB_ID(),
    OBJECT_ID(N'dbo.Heapish', N'U'),
    INDEXPROPERTY(OBJECT_ID(N'dbo.Heapish', N'U'), N'UNIQUE_c2', 'IndexID'),
    NULL, 'DETAILED'
) AS DDIPS;

SELECT
    DDIPS.index_level,
    DDIPS.page_count,
    DDIPS.record_count,
    DDIPS.min_record_size_in_bytes,
    DDIPS.max_record_size_in_bytes
FROM sys.dm_db_index_physical_stats
(
    DB_ID(),
    OBJECT_ID(N'dbo.Heapish', N'U'),
    INDEXPROPERTY(OBJECT_ID(N'dbo.Heapish', N'U'), N'NONUNIQUE_c2', 'IndexID'),
    NULL, 'DETAILED'
) AS DDIPS;


The output is:





Both structures have the same row size at the leaf level, but the nonunique nonclustered index is 12 bytes larger than the unique nonclustered index at the non-leaf levels due to the 8-byte columnstore locator, plus 4 bytes of overhead for the first variable-length column in a row (uniquifier is variable length).