The fee (title) to the property will pass as intestate property according to the state laws of intestacy. You can check the laws in your state at the related question link provided below.
The remainderman would own the property in fee simple. That is absolute ownership. That owner could sell the property or leave it to his beneficiaries in her/his will. If they died intestate (without a will) the property would pass to their heirs at law according to the state laws of intestacy.
In New York State, a life estate expires upon the death of the life tenant. The property will then typically revert back to the remainderman or the owner of the future interest as specified in the life estate arrangement.
This is one of the real challenges in a life estate. State and country laws vary, but in general: The Life Tenant is responsible for the mortgage interest as well as the taxes, ordinary maintenance and repairs. The Remainderman is responsible for major repairs and the principle of the mortgage. One option may be to give up the life estate, leaving the Remainderman entirely responsible for the property, including the mortgage. This could be a negotiating point. I highly recommend visiting an attorney licensed in your jurisdiction to get help that works in your situation.
The estate goes to the state. They will then determine what to do with it. Typically the estate is liquidated, everything sold off and the money goes into the state budget.
In most countries the state takes over the estate and distributes it to the deceased's relatives in accordance with the law. If no relatives can be found the estate becomes the property of the state.
What happens if a life estate is abandoned depends on state laws.
If the person left a Will the Will will identify who the administrator (executor) of the estate is to be - only this person can anministor the estate. If there is no Will then the state will appoint an administrator (there will be country specific laws relating to how this happens).
The estate goes into probate and will be awarded to the state or residency.
It will be distributed according to the intestacy laws of the state. In most cases there isn't an issue with that, as the children will inherit. And the children can open an estate regardless if there is a will or not.
It goes into Probate court and the State decides how it is disbursed.
State probate laws will determine how the deceased's estate is distributed.
Report your suspicions to the Probate Court in which the will is being probated.