My passport has a protective thin transparent case. There's a place to put cards and money on the inner side of the last page. A tiny space. I keep a US$100 bill there. A passport isn't attractive to thieves so that's my rationale for keeping money there. I also keep cash in my wallet and backpack.
Recently I had an unusual interaction with a border agent. I've entered foreign countries in four continents more than a dozen times and never had a rude border agent except this one. This border agent was quite rude. A few moments after getting my passport, they raised their voice and told me to do something in their native language even though they knew I don't speak it. They later spoke in English and told me to take the money out. I did. They used a magnifier of some sort(?) to look at that page.
Was it my fault that I put that money there?
Did I block off something important?
Maybe they thought that bill was a bribe??
What were they looking at with that device? What's on that page?