Sql Server Plan - what is a Filter after an Index operation?
Filter operators are often used for non-SARGable predicates, or when the predicates are too complicated to be pushed effectively to the index access operation.

In your case, I'm willing to bet that it's a series of optional predicates. These are not SARGable: Conor vs. Optional Parameters .


  ...plus some other conditions on some other SP parameters... 


If your code is doing something like col = @var or @var IS NULL, it would fall into that category, and may result in the filter operator.