What table(s)?  CI_Item?

it was part of the common information key, but

CI_Item.M4T

Look for missing Item Codes.  It could be purely corrupted records were removed but I've seen real records removed by the utility too, (which could be handled better by renumbering to remove the "illegal characters").  If it's an item code which was used, you'd typically see similar errors in transaction files (through various modules).

Ok. This could haunt me for a while it sounds like.

I have a solid set of backups.

If I had to rebuild the key and it's data, I could open the old CI_Item locate the removed key and insert a new key?

Possible.

Is rebuilding keys a bad idea?

I was doing a test migration to our new Sage 2016 environment, and wanted to bring clean data.

Rebuilding keys is a good idea, especially before upgrades, but you have to watch the results.  I had one recently where accented French characters caused a couple dozen customer contacts to be removed.  The Sage UI allows the characters, but the utility considered them illegal.

Most of the time it only catches true data corruption (caused by a crash / power outage / disconnection in the middle of writing data to a file), but there are a couple logical loopholes.