This has always bugged me.
In Terminator 2, it is made clear that the terminator that came in the first movie resulted in a changed history, with Skynet being developed at a faster rate by Cyberdyne systems based on research they did on the terminator arm and chip.
This plot point is emphasized during T2 when they blow up Cyberdyne systems, and at the end where Arnie must destroy himself to prevent the same thing from happening.
But the "Uncle Bob" Terminator had to break off his lower arm in some machinery after a tussle with the T1000. Why then were they so oblivious to the arm he left behind in the factory? Why was it not mentioned in any subsequent films?