What tools are there to investigate why my FICO score would have dropped significantly?

Besides the things you mentioned, there are a couple of other things that can cause your credit score to go down:

1. Credit utilization could have gone down.

As JTP reported in this legendary answer, having a credit utilization of zero can be seen as worse on the credit score than having a small but non-zero utilization percentage. It is possible that last month your credit card reported a non-zero balance, and when the balance was reported as zero this month, the credit score could have taken a small hit.

2. Old accounts falling off your credit report.

Even after accounts are closed, they can stay on your credit report for up to ten years, and if they have a perfect payment history they can be positively contributing to your credit score. After they disappear, they are no longer contributing to your credit score and could theoretically cause a drop.

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In my opinion, it is not worth worrying about a score drop of this size to your excellent score. At your level, a 30 point swing in one direction or the other doesn't matter at all. My advice to you is continue to pay your bills on time and don't think about your credit score anymore.

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To answer Fattie's follow-up question in the comments: Because the credit score is calculated based on a proprietary secret formula, it is not possible to determine exactly, specifically what caused the fall. Unfortunately, there is no way to audit the score. However, we do know that the score is entirely based on what is contained in the credit report. So if you have a credit report from before the drop, and a credit report from now, the answer lies in the difference between the two reports.