Is it a bug or a design decision that all life-sign detectors use ONE cool-down?

The description mentions that each uses a unique signal, which I took to mean that they can work independently of each other and that the cool-down was for the individual life-sign detector, not for all life-sign detectors everywhere. However, currently, using one detector renders them all inoperable until the cool-down expires, making owning more than one moot.