Electronic tag

1. An electronic tag configured to be secured to a body of a person, the electronic tag comprising:

an antenna configured to transmit or receive an electromagnetic signal; and

an inductive proximity sensor configured to:

determine a proximity of a metallic object to the electronic tag, based on electromagnetic coupling between the inductive proximity sensor and the metallic object; and

determine, based on the proximity being below a proximity threshold, that a metallic object is attenuating the electromagnetic signal transmitted or received by the antenna;

a counter configured to have a value that is changed towards a blocked threshold in response to a proximity determination being below the proximity threshold and changed towards an unblocked threshold in response to a proximity determination being above the proximity threshold; and

a computer-readable storage medium configured to store a blocked state, wherein the blocked state is set to blocked in response to the value of the counter reaching the blocked threshold to indicate the electronic tag is blocked and the blocked state is set to unblocked in response to the value of the counter reaching the unblocked threshold to indicate the electronic tag is unblocked.

2. The electronic tag of claim 1 , wherein it is determined that a metallic object is attenuating the electromagnetic signal transmitted or received by the antenna in response to both the proximity being below the proximity threshold and the electronic tag being unable to receive an electromagnetic signal.

3. The electronic tag of claim 1 , wherein the inductive proximity sensor is activated either in response to the electronic tag being unable to receive an electromagnetic signal for a period of time, or in response to the electronic tag being unable to receive a reply to a transmitted electromagnetic signal within a period of time.

4. The electronic tag of claim 1 , further configured to determine the location of the electronic tag based on an external location signal, and the inductive proximity sensor is activated in response to the electronic tag being unable to receive the external location signal for a period of time.

5. The electronic tag of claim 1 , wherein the inductive proximity sensor periodically determines the proximity of the metallic object to the electronic tag.

6. The electronic tag of either of claim 1 , wherein the electronic tag is configured to generate a message indicating a change in the blocked state.

7. The electronic tag of claim 6 , wherein the message is transmitted to a monitoring station in response to the electronic tag being able to transmit and receive an electromagnetic signal.

8. The electronic tag of claim 7 , wherein it is determined that the electronic tag is able to transmit and receive an electromagnetic signal based on determining that the proximity is above the proximity threshold.

9. The electronic tag of claim 1 , wherein it is determined that a metallic object is attenuating the electromagnetic signal in response to the proximity being below a proximity threshold for one or more of: a period of time; a plurality of proximity measurements; and a plurality of failed attempts to obtain an external location signal.

10. The electronic tag of claim 1 , wherein the metallic object is a metal foil.

11. The electronic tag of claim 1 , wherein the metallic object is aluminum foil.