Not all airframe modifications airframes are officially approved or noted in the logbooks. Shocking as it may sound, many aircraft owners are tinkerers, shade-tree mechanics or aircraft builders. Some have even been known to make unapproved changes and mods without disclosing them to their own mechanics or the FAA.
When I purchased my 180, there was a lovely machined iPad bracket mounted to an overhead hard point where the windshield and airframe attach (an attachment point appropriate for a visor.) The eight-inch aluminum bracket was obviously custom-machined and not off the shelf. I liked it well enough until the day I realized its extremely sharp corners were a guaranteed cranium opener. I removed it. Since there was no paperwork documenting its installation, no additional paperwork was necessary.
For older planes like the ones I fly, the primary POH has been amended many times over. The pilot flying a modified aircraft like mine must maintain a healthy skepticism about what is in the POH and a questioning mindset about whether the original figures in the POH are still accurate and how they might have changed. Mindful pilots look for mods, check for placards, read the documentation and do their best to grok what any and all modifications will mean to aircraft performance.
When flying a plane that is new to you, be skeptical and look for modified systems. Ask about modifications and look for the logbook entries that show the mod was installed correctly and verify that all placards and POH supplements are present. You could say this is the mechanics responsibility, but you are the one with skin in the game.