You are right to be cautious about this and you should perhaps consider the information you know yourself so that you can make an informed decision about this. Grandparents are relatively recent "ancestors". Do you have parents you can ask about this? Aunts or uncles? Are there cousins of your that might have similar rights? Do your family know about the circumstances of your grandparents life and their circumstances on death? Once you have done this you may want to approach the heir hunters and ask them the extent of the estate they have uncovered and why they think you are entitled. You could also approach the government direct and ask if there is any unclaimed estate led in the names of either grandparent. Any unclaimed estate would end up with the crown and it is easy enough to access those records nowadays. Happy to discuss further. I hope that helps. Please leave a positive rating so that I am credited for my time.
Unclaimed estates are called bona vacantia and are administered by the Treasury in England and by Crown Office in Scotland. Assuming you are in England here is a link you can use: https://www.gov.uk/government/statistical-data-sets/unclaimed-estates-list
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