Secure way to turn battery powered device on/off?

So I'm building up an portable alarm circuit and I'm looking for a secure way to turn it on/off

Originally I looked at Reed switches but these appear to just look for any magnet, so not really secure enough.

I'm now thinking RFID is the only way to go (I want to avoid button pressing), but was wondering if anyone else has any suggestions?

Also I've sourced a RFID reader (RFID-RC522) and it's unclear what ma it operates at. Reading the datasheet the peak of 30ma would deplete my battery in under a day but if it waits for a card in it's sleep state that wouldn't be a problem (it uses minimal UA in this state). Anyone got any experience with these?

Unless there is something else to activate it, I don't see how it can wait for a card in its sleep state. The card reader would have to keep broadcasting its radio transmissions to see if anything reacts to it. Unless there is something else to activate it, it would have to keep doing this, all of the time.

A solution to this, would be to have some other detector which notices your approach, and only when it notices that you are there, it will turn on the RFID reader to attempt to read your card.

Yeah that's what I thought, although what's the point of a low power state if it can't detect a card surely you'd simply turn the detector off at that point.

I guess I could combine it with a light or PIR sensor to activate the reader when required.

So I've done a bit more research... what about an iButton? Specifically DS9092

I've done a bit of research and checked the datasheet and can't seem to find a rating for how much amps they draw when idle, anyone know roughly how much they draw?

Perhaps you could combine a reed switch with RFID or some such. The reed switch wakes the thing up but it won’t do anything unless it receives a suitable code within a preset time.