Teardown

First of all: The tracker for that achievement is working.

There are far more than 200 valuables in the game.
When i finished the story, i was over 90% just by looting stuff i found while exploring the maps and destroying stuff. Valuables do not trigger alarms or a timer of any kind, but they add money to your account that can be used for upgrades. So, to make your life easier and the completion faster: I would suggest you go for them right away while doing the story.
Jetpack-Mod is a huge help and i suggest you use it, but it's not essential.

Valuables are saved as soon as you pick them up. You can leave the mission when you're done cleaning up missed items.

As i said, you'll probably find most of them on your own... but here's a visual guide for (all) of them if you have to clean it up. https://steamcommunity.com/sharedfiles/filedetails/?id=28337...