Which of the following could be a potential issue with microwave detectors?

Microwave detectors operate by emitting microwave signals and analyzing the reflections off objects within a certain area. A potential issue with these detectors is their sensitivity to moving metal objects. This is because moving metal can interfere with the microwave signals, leading to false positives or misinterpretation of signals. When metal objects move, they can cause fluctuations in the returned microwave signal, which may confuse the detector and result in unintended alerts.

The other options highlight characteristics that are not typical of microwave detectors. For example, they are generally sensitive to movement, can detect various speeds of objects, and do not have unlimited range; their effectiveness diminishes with distance and obstacles in the environment.