The concept of "never" can also be an unwise assumption. Many owners of antiquities take comfort in the belief that even if their items were looted, unless those items appear in incriminating evidence (such as the Medici or Becchina archives, or other documented sources), their illicit origins will never be detected.
But the word "never" here takes no account of the rapid pace of advances in technology and methodology, and their application in fields such as archaeology, anthropology and forensic science. Scientific dating methods, for instance, progressed immensely in the 20th century; since dendrochronology was developed during the first half of that century, the technique of radiocarbon dating was added to the arsenal in the 1940s and procedures such as thermoluminescence (TL) dating, archaeomagnetic dating and racemisation dating joined it during the 1960s. With the continuing development of dating methods and their alliance with further headway in fields such as geophysics and soil science, it is far from unrealistic to foresee a time in the future when it may be possible to determine not only in what region an artefact was buried but also how long it has been UNburied, i.e. the length of time that has elapsed since it was released from an environment compositionally different to that of the atmosphere (a typical soil, for example, is relatively low in oxygen and high in carbon dioxide).
In that event, scientists would be able to establish with some degree of probability in a lot of cases both where an artefact was excavated and when it was excavated. And thus the illicit origins of many looted antiquities would indeed be detected, regardless of whether the looting was documented in incriminating evidence or not.
It's all hypothetical at present of course - but the thought may spur buyers of antiquities to be less complacent about acquiring items that may have been looted. As Gandhi once said, "The future depends on what we do in the present".