This is a magnet detector and it’s currently detecting a magnet, pointing at the pole.

It can detect magnets through a door quite happily. This includes the kinds of magnets used in door contact sensors. I can then place a larger magnet in the right location and open the door; your alarm or access control system will think the door is still closed.

These larger magnets are, frankly, a pig to carry around because they stick to everything and you may need a few of them. I have a switch magnet (you can turn the field on and off with a large switch) which makes it more convenient, but they’re still big and heavy.

So how does all of this information help you? Simple – on important doors, add some extra magnets. I won’t know which one is actually connected to a sensor (unless I’ve seen the other side of the door, and even then it may not be obvious) and so I’ll have to deal with all of them. This, of course, assumes I have bothered to look all around the door and not just assumed I’ve found the sensor when I hit the very first magnet. Dealing with them all is perfectly doable, but it’s another obstacle to overcome and another opportunity for me to make mistakes, deviate from my plan or need to come back with more equipment.

Oh, and if you’re wondering if a magnet can truly trip the sensor through a door – yep. The one I use will trip a reed switch on the other side of a door at around 6” from said door.