In order to make a "claim" against your current home, the owner of the debt for the mortgage on your other home would have to get a judgment against you and record the judgment with the recorder of deeds to create a judgment lien against your home. Once the lien is created, they can levy to collect against the lien. That action would take the retirement property out of your name.
If and when they sue you, you should talk to an attorney about whether you have any defenses to the amounts being claimed. The mere effort to collect the money by a collection company can not in and of itself create a lien against your property - it has to be based on a judgment.
This answer does not constitute legal advice and does not and is not intended to create an attorney-client relationship. The law may vary depending on the state in which you reside. It is intended only to give some direction in which to seek assistance.