Your cargo tank is placarded for Division 2.1 (flammable gas) and is currently empty, having just been unloaded. A coworker lights a cigarette 20 feet away from the tank. Is this a violation?

49 CFR 397.13; AAMVA CDL Manual Sec 9.6.6 (nhcdm.pdf p.9-14)

Residual vapors in an "empty" flammable-gas or flammable-liquid tank are often just as dangerous as a full one, since there is no liquid bulk to absorb heat and vapor concentrations can actually be higher in the empty headspace. Treating an empty placarded tank as safe to smoke near is a common, dangerous misconception that the rule specifically closes.