I think the stupid tests were wrong.
A case of media pornography.
Sprinklers are used by the landscaping industry. That industry also uses nitrates for fertilizer. The same chemical family of nitrates used in C4 (McVeigh used nitrate fertilzer as did the rags that blew up the WTC garage in 1993).
The test came back positive for nitrates, some "journalist" asked what that meant and the Port Authority representative says, "it can be used in explosives like C4."
Voila! C4 is much more "pornographic" than "fertilizer".