Extremely rare technical term, possibly from older firearms or mechanical contexts. Not used in everyday Japanese; the modern equivalent is サプレッサー.
The term 隠蔽子 sometimes appears in old documents as a part of firearms.
Compound of 隠蔽 (concealment) + 子 (small thing/device). The exact origin is uncertain; it appears to be a coined term from the early modern period, possibly a translation of a Western firearm component.