Hello @Lordakurei, welcome to the community!
Did you check the decay timer for those placeables with a repair hammer, and were there any relevant messages in the event log?
If they were not placed over foundations, its possible that the decay timer had a lower maximum value than expected, but it’s also possible that they were destroyed by either a Purge or even NPCs.
It’s advisable that all placeables are built on top of foundations linked to the main base so that the decay timer is shared between them, as building pieces and placeables may decay within a matter of hours if they’re not interconnected to a larger network of other building pieces.
For further clarifications on this mechanic, you may read the following article and check out the video:
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