Butane is a subtle odor and taste, so as noted is easily masked by the unsubtle monoterpenes. It's more detectable extracting old material mostly devoid of the competing aromatic odors and flavors.
Here is a link showing sensory thresholds for different solvents when isolated. https://cschi.cz/odour/files/world/M...g%20Method.pdf
It shows butane and propane at 1200 and 1500 PPM/VV for odor.
As a GRAS solvent with an FDA 5000 ppm residual standard, and such a high odor sensory cutoff, the issue is taste rather than toxicity and odor.
Our sense of taste is more refined, but sorting out the butane flavor from the monoterpenes below 500 ppm is a challenge for me, and our cotton candy concentrates typically test below 300 ppm after about 4 1/2 to 5 hours under vacuum at ~115F, with a couple flips.