Barely perceptible.

He wrapped the corner into the heart of the handkerchief, and put it back into his pocket.

Some powerful narcotic had been forced into the room through the keyhole.

This would account for the prince's headache.
These Orientals were as bad as the Dutch; they never opened their windows for fresh air.
Beyond this faint, mysterious odor there was nothing else.

The first step would be to ascertain whether this narcotic was occidental or oriental.
"Nothing doing yet," he confessed to the anxious manager.