Maybe a dumb question, but why would this warrant an evacuation? I get that it's contraband, but it's drugs, not a weapon. Unless they thought since one disallowed thing made it's way in that there could be other stuff (weapons) as well?
Edit: or maybe they thought it could be anthrax at first, which makes much more sense as to why they would evacuate
A lot of chemicals are white powders, including stuff like anthrax. When you find an unidentified white powder, especially in a place like the White House, you should always assume the worst until you can get it identified.
Source: am chemist who teaches a bunch of college slobs that are always leaving the lab a complete mess.
Maybe it's just because I'm an American that remembers 9/11 and the anthrax scares that came soon after, but I completely understood the evacuation because of a "mysterious white powder".
Everyone was evacuated to the bathroom while the substance was safely eliminated at which point they all talked over one another discussing their sure fire business plans and how awesome Led Zeppelin is.