Also known as Budder, which was more popular before vacuum finished waxes.
At 20 minutes purge, you may be below your own sensory threshold, but you still have retained butane. If you don't believe it, put it in a -29.5" Hg vacuum at 115F and watch the solvent bubbles escape.
Even at 115F and -29.5" Hg, it still takes 30 to 45 minutes for a full purge.