I'm voting that a shorted lamp diode has been missed on that light board, or on the "21" lamp which is on the same column.... could be other lamp diodes causing it than just the Lock 2 and PC inserts.

Here is a video on Matrix and Diodes that helped me alot to understand these wierd problems. It is a Switch Matrix video but the principle is the same just one is an input the other is an output. Among the most useful 10 minutes of you tube I have ever watched! Lol

Go to lamp tests, get yourself a photocopy or a snapshot of the matrix to doodle on and mark down all wierd associations in Single Lamp Test (when there should only ever be one CPU lamp lit at any one time). The map you create will tell you some story and where to look and test. Draw an all encompasing loop/circle around lamps that are lit together. Colour code for different lamp tests if it helps. You should find some indicators in the odd behaviour.

Hope this helps.

**Also maybe give this a go: http://pinballmatrix.com/

Also note that to expose some lamp matrix issues you may well need to use the "Rows Test" and "Columns Test", so that you can lite all lamps simultaneously in a given column or row. Unfortunately the game won't let you light full columns an rows together, but row and column tests is better information than only single lamps.