One day, as I went from store to store, I kept setting off the alarm every in every shop I visited. In each case, they waved me through because I guess I don't look like their stereotype of a shoplifter. Eventually I got to the inkjet recycling store and handed over the old cartridge that I had been carrying, and the alarms stopped going off.

The next day the supermarket had a security guard by their alarm, so I told him about the inkjet cartridge the day before. He acknowledged that they often set off the alarms, but said that some brands of mobile phone were even more problematic, so it's quite possible that a tablet could set them off too.

I don't think Bluetooth is anything to do with it; the alarm is picking up a re-radiated signal caused by some kind of resonance and nonlinearity (which generates harmonics) in some circuit of the device.

I have now learned how to stop my inkjet cartridges setting off the alarms. The main trick is to walk right in the middle of the doorway rather than next to one of the two sensor towers. It's also important to walk at normal walking speed or slower, as a brisk walk seems to set it off every time. Finally, I have found that at my supermarket one of the two entrances is less sensitive than the other. Now I no longer set off the alarm, except when I forget to walk in the middle.

I suppose when these alarms are designed they are carefully tuned to have minimal false alarms, but in realistic use without regular maintenance they become less reliable.

Roger