The Monkey Magnet sensor is a separate sensor mounted on the backboard near the base of monkey mech, it's purpose is to detect when a ball is actually captured by the Playfield magnet. It does this by sensing a larger magnetic field when the ball is on the magnet. It is very common that this sensor needs to be adjusted or moved to make it more accurate with regards to sending the Monkey down to get the ball. In other words, when all is working as designed a weak shot that triggers the magnet (via rollover switches in the loop) but does not get captured by the magnet on the PF will not trigger the Monkey to travel down and it will NOT award a lock. The code obviously compensates for when it's not working properly by awarding the lock even though no ball gets deposited in the castle. There is a specific test in the menu (Page B-18 of the manual) for this sensor and if you run those tests with a ball you can move the sensor around until you find a spot that is more reliably accurate. A few things to remember; you may have to move it pretty far and there is a lot of stuff (monkey mech) in the way back there, don't let the magnet stay energized too long as it can get quite hot and finally patience is a virtue - mine was not easy to find a good spot and the first couple tries worked for a while and then I had to move it again. Since the last time though it works very well and I very rarely have any false captures.

Here's a pretty good video someone did for this and is shows the sensor, etc.:

The video is helpful but I found it to also be confusing for a couple reasons - the biggest is that you can't see the ball and the screen at the same time to know when sensor should be active. However the overall problem is that the PF magnet itself triggers the ball sensor so it always thinks a ball has been captured when it turns on - this is exactly what my game did when I got it. The first thing you need to do is go into the test and see how much the sensor needs to be moved before the magnet no longer triggers it, then start fine tuning it back toward the magnet until you find the sweet spot / orientation. Remember to not let the magnet get too hot, to let it cool down completely and retest once you think you have the sweet spot established, and then to test again when you put the PF back down in the game. I used some thick double sided tape to temporarily position the sensor when testing.