Small critters are the usual suspect for the motion detector triggering the light when you don't see any reason for the light to be turned on. Reduce the sensitivity and range on the motion detector so it will only turn on when a large object approaches close to the sensor. You can also try repositioning the sensor slightly higher so it will miss small animals on the ground, but be careful not to position it so high that it misses children or adults who may walk by it. Rain wind and snow also effect these light and their sensor simply wear out too quickly. Some models of the Zenith brand were covered under a lifetime warranty but I can't say which models that were covered, so if you have your reciept and warranty you might want to check on that. The security sensors are replaceable with a model sl-5315 which can be purchased at the local hardware stores.
Here are a few things for you to check: 1) make sure your unit is sealed. Any holes or openings in the unit's casing will affect operation. 2) you may need to readjust the unit's sensitivity. There may be an adjustment pod or jumper inside the detector to reduce sensitivity. 3) make sure the unit is mounted securely. If the wind causes the detector to move, even slightly, this could be the cause of your problem.
Are there any other lights nearby that could be shining on the photocell of the sensor light? If the controller thinks that it's daytime, then the LED will flash to show that it is detecting motion, but the lights won't come on. A nearby streetlight, bright light from a window, etc. can confuse the controller. Here is the web support site for Heath Zenith
http://www.heath-zenith.com/servsup.htm
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