OR <column name> IS NULL does not prevent index seeks

(yes, I used an IN rather than a NOT IN, the NOT IN would have been a scan because of the size of the table and the distribution of data in it)

What can easily lead to scans is if you have something like WHERE Column1 = Something OR Column2 IS NULL

http://sqlinthewild.co.za/index.php/2011/05/03/indexing-for-ors/

Also, a NOT IN may well be evaluated as a scan whether or not the where clause is SARGable, because of the portion of the table that will be returned.

Be aware that the alternative with ISNULL is no longer SARGable and hence can't seek on an index even if the volume of data would allow it.