Is 几号 a slang word for heroin? Looking up 几号 in the Pleco dictionary returns a definition different from what one might expect from the usual meaning of the phrase. 
Is 几号 really used as a slang for heroin?
Is 几号 really used as a slang for heroin?
Yes.
百度百科｜海洛因（吗啡类毒品总称）

海洛因，系列吗啡类毒品总称，是以吗啡生物碱作为合成起点得到的半合成毒品，俗称几号、白粉、白面。


Here is a link to the Change Log Entry for this word in CC-CEDICT. It was submitted with the following evidence:

海洛因，系列吗啡类毒品总称，是以吗啡生物碱作为合成起点得到的半合成毒品，俗称几号、白粉、白面。
https://baike.baidu.com/item/%E6%B5%B7%E6%B4%9B%E5%9B%A0/544824
(slang) heroin (powerful and addictive drug)
https://en.wiktionary.org/wiki/%E5%B9%BE%E8%99%9F

CC-CEDICT has a fairly strict vetting process. Entries are first submitted usually with a certain amount of proof of usage, references, etc. Afterwards there is a review process where editors will go over the submission - sometimes requesting additional evidence - or adding documentation that they themselves have come across while attempting to process the entry.
Of course mistakes can happen, but you'd be hard-pressed to find recent outright errors in CC-CEDICT.

Apparently 几号 comes from the fact that there is:

海洛因1号、海洛因2号、海洛因3号、海洛因4号

Different levels of purity in heroin.