Reading from a declassified portion of a report by the National Ground Intelligence Center, a Defense Department intelligence unit, Santorum said: "Since 2003, coalition forces have recovered approximately 500 weapons munitions which contain degraded mustard or sarin nerve agent. Despite many efforts to locate and destroy Iraq's pre-Gulf War chemical munitions, filled and unfilled pre-Gulf War chemical munitions are assessed to still exist." - From Fox News, emphasis mine.
The key thing to notice here is that the US has only found degraded munitions in Iraq. That is to say "ex"-WMDs.

The Defense Department knew that Iraq's chemical and biological weapons stockpiles were degraded well before 2003, and that regardless of the continued existence of the shells, they were useless as weapons. Hence, they knew that Saddam didn't possess weapons of mass destruction, but that he might still possess some ex-weapons of mass destruction. Ex-WMDs are not particularly threatening to US security, and were unlikely to scare US citizens into supporting the invasion of Iraq. That is why they talked about unaccounted for stockpiles, but failed to mention that they knew that those stockpiles were useless as weapons.

See how honest they were being in the run-up to war?

According to the Defense Department's own experts, Iraq used crude production techniques for its sarin and tabun nerve agents. They had a shelf life of five years. When shells were found during the first Gulf War, many of them were already leaking. Saddam's anthrax stockpiles would have degraded within three years. Thus, the DoD knew that any Iraqi stockpiles that had survived the inspectors (and the reported destruction of stockpiles overseen by Saddam's defecting son-in-law) were completely useless and non-threatening.

That's why this Republican talk of having "found" WMDs is just more silliness. After a massive effort, all they've found is a few degraded ex-WMDs from the pre-Gulf War period. Not a threat or a rational rationale for war.

Check out:
The Militarily Critical Technologies List Part II: Weapons of Mass Destruction Technologies (ADA 330102), "Chemical Weapons Technology" - U.S. Department of Defense, Office of the Under Secretary of Defense for Acquisition and Technology, February 1998 (updated in 2002).

Do a PDF search for "Iraq" to read the details on the honest assessment of their weapons program by the DoD in '98 and '02.