Reverse affect alarm

I have a cheap door alarm at home (get's the job done!), it helps to warn me when my 4 year old has opened the door...

I wanted to work on a project that would reverse the affect. For example, it would sound the alarm if one side of the unit is out of range (10 foot range with urban obstacles like walls, trees, etc...), with the possibility to increase/decrease its range.

What do you guys suggest.

Thanks in advance! or:

Magician: or:

I was hoping for a solution that would activate when out of a certain range, for example ?5 feet = alarm in standby, >5 feet = activate alarm. A sort of set it and forget it solution, like the door alarm sensors that would only require a natural reaction of opening the door to go off.

These are just simple lost key finder...

At least the second one is a proximity sensor. You can set the range and an alarm goes off if your pet/kid/handbag goes outside the set radius.

I guess you could hack one side to attach to your ardu' rather than/as well as setting off the inbuilt alarm.

Specifications: Credit Card Shape Anti Lost Alarm with Li ion Battery Included Li ion battery inside for lasting up to 15 days standby time Makes loud alarm when object is out of the setting distance

Safe distance: 2 10m (If there are no special wireless blocking) Distance setting: 2 3m / 5 6m / 8 10m

I included second link if you want to find better price for such device.