[During the preparatory work] it was agreed quite early on to propose two periods, reflecting the rules that obtain in the different legal systems: one relative (running from the time when the claimant has actual knowledge of the object’s location and of the possessor’s identity), the other absolute (running from the time of the theft). The length of the two periods was however left open – this being an issue regarded as central to the package constituted by the Convention as a whole that should be settled by the diplomatic Conference in relation to the other parameters of the text. The draft accordingly confined itself to proposing the upper and lower limits contemplated in the preparatory stages, namely one to five years for the relative limitation period, and 30 to 50 years for the absolute period.
The relative limitation period refers to the situation inwhich the claimant has sufficient information to bring an action, that is to say, it has actual knowledge of the location of the object and of the identity of the possessor. In such cases, the claimant has three years within which to bring its claim for restitution [and return], this being regarded as the minimum needed to collect all the evidence on account of the object’s being located abroad – but also long enough since the period does not actually start to run until the claimant has knowledge of these two key elements. A one-year limit was deemed too short for use in an international instrument, and while three years is shorter than the period required by some legal systems, the time when the limitation period starts to run should be borne in mind as should be the fact that Contracting States are free, in accordance with Article 9, to continue to apply their own limitation periods or to introduce longer periods.
The information required of the claimant to enable the period to start running – knowledge of the object’s location and the possessor’s identity – is cumulative. This was seen as being in the claimant’s best interest: mere knowledge of the possessor’s identity would be insufficient since the latter, were it to become aware of having been discovered, could then remove the object and make it disappear.
Unlike the rules relating to theft, which stipulate an exceptional limitation system for claims for the restitution of certain categories of objects (those that form an integral part of an identified monument or archaeological site or belong to a public collection – cf. Article 3(4) and (5)), or that belong to or are used by a tribal or indigenous community – cf. Article 3(8)), Chapter III makes no such provision, on the grounds that the objects in question are by definition always stolen before they are illegally exported.
Extract from Schneider, UNIDROIT Convention on Stolen or Illegally Exported Cultural Objects : Explanatory Report, Unif. L. Rev. 2001-3.