Why is there a scan on my clustered index?

A clustered index scan is how SQL Server designates a full table scan on a table with a clustered index. This is because you don't have enough indexes on the SIGN table to satisfy the WHERE clause, or because it decided that the SIGN table is small enough (or the indexes not selective enough) that a table scan would be more efficient.

Just by examining the query, you'd probably have to index the DeptID column as well as some combination of StartTime, EndTime and NEDStatusID to avoid the table scan. If the reason you're asking is because you're having performance problems, you can also run the Index Tuning Wizard (now the Database Engine Tuning Advisor in the SQL2005+ client tools) and have it give some advice on which indexes to create to speed up your query.

Because your WHERE clause isn't against indexed columns.

Here's a good blog post about when SQL Server reaches the "tipping point" and switches from an index seek to an index/table scan:

http://www.sqlskills.com/BLOGS/KIMBERLY/post/The-Tipping-Point-Query-Answers.aspx

You may want to look at the way your queries are filtering, as the tipping point is often much fewer rows than people expect.