Hy,
since 1.24 version that exists a feature for creating events when the camera signal is lost:
ZM_SIGNAL_CHECK_POINTS
For locally attached video cameras ZoneMinder can check for signal loss by looking at a number of random points on each captured image. If all of these points are set to the same fixed colour then the camera is assumed to have lost signal. When this happens any open events are closed and a short one frame signal loss event is generated, as is another when the signal returns. This option defines how many points on each image to check. Note that this is a maximum, any points found to not have the check colour will abort any further checks so in most cases on a couple of points will actually be checked. Network and file based cameras are never checked.
There's a way to use this features with network cameras also?
Thanks!