Statute of limitation would make the country of Poland the rightful owners.
There aren't statutes of limitations about this in international law, as we've seen with the recent Munich art collection.
What it almost certainly isn't, is the Amber Room. That was last seen in Kaliningrad and although there have long been rumours that it went to South West Poland, there's a. a lot of wishful thinking there and b. probably not much left given the state it was in when it was removed from Tsarskoe Seloe and subsequent deterioration.
Some things are still missing - the national silver reserves of Hungary for example. If it contains that, then it is legally theirs. If it contains gold plundered from concentration camp victims, the state would have a moral duty of restitution where possible - they couldn't just keep it without trying.
The same with artwork. If there are owners or heirs of whatever nationality, it would need to be returned; although the condition of such artwork would probably mean that either there isn't much left or it would need long and careful restoration.
If it contains items that are unclaimed or genuinely untraceable, it would only then be the property of the state.
And they have warned that it is likely to contain rusting military equipment.