Your New Relic synthetic monitor reported a failure from a single location, but the application appears to be up and running, so you're not able to identify the problem.
A single failure from synthetic monitoring is not as limited as it may seem. It represents three consecutive failures of the same monitor. When a monitor fails, New Relic immediately queues two additional checks for the same monitor and location. New Relic reports a failure only when all three jobs fail. (Only the third monitor run is visible in the Synthetics UI.)
The failures do not need to be the same error type. This further reinforces limitations on false positives.
Synthetic monitors may sometimes fail from a single location when the site appears to be running properly from other locations. While these failures are isolated, being able to identify issues specific to a particular location can be very valuable.
Here are some techniques to diagnose possible issues behind isolated monitor failures.