The magnets will set off metal detectors, because metal dectors detect any change in magnetic field.
Security detectors, however, work on two different principles. The first one is that the gate has a weak field between the two posts, and this field will cause the tag to start resonating at a specific frequency when it is moved inside of the field. The detectors will pick up on this frequency and the alarm will sound. The pads that deactivate these tags will use magnetism to adjust the frequency emitted, hence making them invisble to the gate. (I have had some cases where mechandise that I bought from one store would set off another's alarm, because they used opposite frequencies for on and off).
The second, newer kind is using RFID technology, which allows to gate to not only detect, but identify the merchandise.
Both of these should not respond to magnets, a fact others have already testified to.
For more info, here is a
website that explains a bit more.
At least I get to use my physics knowledge for something productive.
-James Physit