The pulse scanner was a stationary sensor that automatically sends out an alert when it detects nearby enemies. Presumably, it functioned the same as the Orca's rechargable pulse scan, since it could detect stealth units. However, it required several seconds to transmit the alert, during which it could be destroyed.
In 2034, the Steel Talons placed a number of these pulse scanners near Marcion's stronghold in Australia. However, Marcion's Black Hand somehow found a way to bypass them, allowing his forces to patrol near them without triggering their alarm.
When loyalist Nod forces led by LEGION attacked Marcion's compound, they encountered these pulse scanners, and LEGION was instructed to destroy them to prevent them from alerting the Steel Talons. Should LEGION fail to do so before destroying Marcion's stronghold, the Steel Talons would attempt to intercept Marcion's convoy and kill him during his escape.