At the risk of answering a rhetorical question, No, you probably shouldn't blindly trust someone that may not know all of the same facts as you. However, you don't mention with any specificity what your grandfather told you, or whether or not you've shared this information with the administrator.
First, make sure that the administrator knows what you know. The obvious next step in that process is for the administrator to actually verify what you've been told by your grandfather. Second, be aware that in many cases a significant part of a decedent's assets might pass outside of their probate estate. Life insurance proceeds, retirement accounts and similar assets are typical examples of these. Above all, maintain as much communication with the administrator as you can. Familiarize yourself with the Court's file, so that you can see for yourself what is being reported and what isn't. That way, if things don't turn out how you think your grandfather intended, you're armed with what you need to present to an attorney to advocate your interests.
This answer does not constitute legal advice. I am admitted to practice law in the State of Texas only, and make no attempt to opine on matters of law that are not relevant to Texas. This answer is based on general principles of law that may or may not relate to your specific situation, and is for promotional purposes only. You should never rely on this answer alone and nothing in these communications creates an attorney-client relationship.
Kindly note and keep in mind that my response is merely a general comment on the law related to your post, and NOT intended to be construed as legal advice or a legal opinion. Also, your question and my response to it does NOT create an attorney-client relationship between us. You cannot rely upon what I have written, because I do not have all of the necessary information that I need to advise you or render an appropriate legal opinion. Even simple facts that have been omitted can completely change my answer. For an attorney to give you legal advice or opinion, you would need to hire him/her to be your lawyer, and then the two of you would need to discuss the matter in greater detail and go over the documents carefully.
In Washington state, if your grandfather's estate is valued at less than $100K, then a successor may use a small estate affidavit to transfer assets, provided, of course, that there is no transfer of real property involved. Assuming that there are multiple successors who stand to inherit assets from the estate, then the successor should mail a copy of the death certificate and the estate affidavit to each successor notifying them of his intent to claim his respective share (or in the alternative if a successor wishes to claim the property on their behalf, then have them each sign a declaration under penalty of perjury). The next step is to present the affidavit, along with the death certificate and declaration under penalty of perjury to the person in possession of the property to be claimed, i.e. the financial institution, bank, brokerage firm, etc. Finally, mail a copy of the affidavit along with the attachments and the decedent's social security number to DSHS (if in WA state).
As always be sure to consult with an attorney licensed to practice law in your state. Good luck!
You have to ask yourself -- what motive would the personal representative (PR) have to not list some of the assets of your grandfather as probate assets? Often, this means that the bank accounts that you thought were just titled to the deceased person, were in fact jointly titled to the deceased person AND to the personal representative. If that is the case, then all those accounts now belong only to the PR. Grandfather's mind may have grwon dim near death, and he may have forgotten that he titled those accounts jointly with whomever. It could even be that he never intended or understood that the way he titled those accounts would result in the whole thing going to the joint owner at his death. But for you to prove in court that the PR twisted grandpa's arm and used "undue influence" to force or trick him into titling those accounts in joint names is a VERY difficlut and expensive thing to so. I suggest asking the PR if he or she will just tell you whether that "missing money" was actually in jointly titled accoounts with the PR. If he or she says NO, then maybe you can share any information you have with him/her so that the PR can track down any truly "missing" money. If the PR refuses to talk with you about this, you can alwayts challenge the Account (not the Inventory) filed by the PR by filing an Exception or Opposition to the Account, and then you can conduct "discovery" -- where you are permited to ask the PR questions in writing to which she must provide written answers, and also to request documents. You will need an attorney if you want to accomplish anything that way.