49 CFR 173.29 contains the PHMSA/USDOT regulations for the transportation of empty packagings that last contained hazardous materials.  As a shipper of hazardous materials you must be aware of §173.29(a) which states that an empty packaging with the residue of a hazardous material will be regulated the same as a full container:

Except as otherwise provided in this section, an empty packaging containing only the residue of a hazardous material shall be offered for transportation and transported in the same manner as when it previously contained a greater quantity of that hazardous material.

There are several methods to avoid full regulation under the HMR when transporting an empty packaging, one of them is detailed in §173.29(b)(2)(ii).  It allows for the transportation of empty packagings not subject to the HMR if the packaging:

Is sufficiently cleaned of residue and purged of vapors to remove any potential hazard;

In addition to cleaning and purging vapors, any material remaining in the packaging can not meet the definitions in §171.8 for a:

• Hazardous substance,
• Hazardous waste, or;
• Marine pollutant.

If this is accomplished you still must comply with §173.29(b)(1) which requires you to complete one of the following:

• Any hazardous material shipping name and identification number markings, any hazard warning labels or placards, and any other markings indicating that the material is hazardous (e.g., RQ, INHALATION HAZARD) are removed, obliterated, or securely covered in transportation.
• The preceding hazard communication methods may remain if the packaging is not visible in transportation and the packaging is loaded by the shipper and unloaded by the shipper or consignee (ie. the receiver of the empty packagings).

So, if the packagings are truly empty to where none of the HazMat remains you are required to either remove all indications of a hazardous material or ensure that the hazard communication methods in use are visible to no one but the shipper and the receiver.

What if the hazard communication methods remain on an empty packaging (cleaned of residue and purged of vapors per §173.29(b)(2)(ii) and are visible in transportation to someone other than the shipper or receiver?

That would be a violation of  §173.29 & §171.2(k) which reads:

No person may, by marking or otherwise, represent that a hazardous material is present in a package, container, motor vehicle, rail car, aircraft, or vessel if the hazardous material is not present.

You know the regulations are complicated when even the shipment of an empty packaging has so many restrictions and requirements.  If you ship empty packagings of hazardous materials, then it is certain that at least some of your employees are HazMat Employees and require the triennial training that I provide.  Contact me for a free HazMat Employee or RCRA training consultation.