The Secret Service says it discovered a white powdery substance Sunday night in the West Wing during routine rounds. It's unclear where exactly in the West Wing it was found -- but a spokesperson for the agency will only say agents stumbled upon it in a "work area."

The SS rep also notes any notion the substance might've been found in the executive mansion is false. While Secret Service won't disclose where the alleged coke lay, it appears DC's FEMS let the cat outta the bag ... reportedly relaying via dispatch it was in the library.

POTUS was not at the White House during this entire ordeal, BTW.

The discovery of the unknown powder prompted an increased security alert and a brief evacuation of the premises, and tests are being done to decide what it might be. WaPo, citing sources familiar with the matter, says preliminary tests suggest it is, in fact, blow.

Speaking of that dispatch audio from earlier ... it seems a firefighter who was on the ground firmly believed cocaine was the culprit, -- telling the department's hazardous materials team that a test they conducted indicated "cocaine hydrochloride." No ambiguity at all.

Officials are saying the substance did not pose a threat, and an investigation is underway as to how it made its way into the White House ... and, obviously, who brought it in.