Actually the answer is fairly simple in the U.S. Unlike copyrights or patents, trademark registration rights can last indefinitely if the owner continues to use the mark to identify goods or services & thats the key since if the owner has been out of business for 10 years the Federal Trademark registration expires & even then if its less than 10 years between the fifth and sixth year after the date of initial trademark registration, the owner must file an "affidavit of use" and pay an additional fee to keep the registration alive plus also file an affidavit and pay a fee within a year before the end of every ten-year period.
So what does that mean? Most of these molds have been sitting around for alot longer than the 5th & 6th year pertaining to the "affidavit of use" because the "owner" has been out of business longer than that (many times its decades)so its fair game with little rules.
Thats the legal answer, the practical answer is if the owner has been gone for years its usually a case of who cares & who has a legal right to do anything about it? So the rule of thumb is know what you are buying before you buy especially if the original company has been out of business for years & know who has the molds & who is reproducing what by whom. Its part of the tuition process.