Abandoned ship systems disappeared?

[PvK]

Generally things seem to stay where I left them, but I recently saw two ship systems that I left behind, not be there when I returned to get them.

One was a system left in debris when I destroyed another ship. My cargo was full, and I expected it to still be there, but when I came back from base, it wasn't there any more. Does docking clear ship components out of space, or did an actual ship or something come and take it? (The trade goods I leave hanging in space seem to always be there when I come back.)

The other ship system I noticed vanish was in a secret mercenary base. My team already had a heavy item, so I took that back to the shuttle and returned, but the ship system was no longer in the base. Is there just a chance (100%?) for them to get snatched, or is there some other mechanic that determines whether a ship component in a cave will be there on return?