But on the up-side, flying the same aircraft means that you do spot when something changes.
Some years back was once in an R44 with another pilot on climb-out when there was a solid loud metallic "thud" (like someone dropped a bag of tools in the rear footwell). Nothing more, no vibration or loss of power - all smooth. We looked at each other, and agreed immediate return. Engineering inspection followed but couldn't find a fault, told to hover it "heavy" to see if it would repeat. 15 min and no repeat, aircraft released to service but not with me in it. A couple of days later another pilot with commercial passengers onboard experienced the same "thud" in level flight. Made a PAN call and landed, aircraft recovered by road followed by head-scratching in engineering. Someone suggested looking at the Sprag Clutch, which requires disassembly and most shops cannot do. When disassembled it was destroyed internally - fault found.
Taught me one thing, if something strange happens and they can't find the fault, then the fault is still there.