In many cases the heir hunter will know the size of the estate before asking people to sign agreements.
No, they won't. As far as I'm aware, they have no idea what's in the deceased bank accounts. They can only make an educated guess based on property owned and maybe what someone did for a living. As I said in a previous post, F & F didn't persue my half-brother's estate, and I can't blame them, because he lived on his own in a one-bedroomed council flat in SE London, he didn't have a phone, and had been in retirement for more than 20 years. As it turned out, his estate was in excess of £180,000 because of the savings, insurance and pensions that he had.