A radar detector is basically a microwave receiver. In today's modern world there are many sources of microwave signals. Automatic door openers at supermarkets, commercial alarm systems such as those found in banks, and cellular phone antenna towers all emit signals in the microwave frequency range. In most cases, the false signal your unit is picking up can be attributed to one of these causes. In addition, road construction areas may utilize a special transmitter that intentionally causes an alert, in an effort to persuade drivers to slow down.
Although some of these systems operate at a different frequency than a radar detector, they do produce out of band stray signals that can cause falsing on your radar detector. Unfortunately, many of these signals are strong enough to overcome the sophisticated Anti-Falsing system built into your Cobra Radar detector. You can reduce these false signals, when driving in urban settings, by placing the unit in the City Mode. Return the unit to the highway mode when driving in rural areas or on the highway.
A detector that does not false is most likely not picking up radar signals and may need to be serviced.