There are several different manufacturers of sensors designed for auto security systems (car alarms) which could very easily be used with a buffered parking mode enabled dashcam in order to trigger recording from the internal buffer to storage card, but I will list a few from Directed Electronics (aka DEI) because their manuals are readily accessible on the internet, and also because they also manufacture some of the hardware sold under different brand names.
Most external sensor modules, regardless of type or manufacturer, tend to interface with an alarm system using the same mechanism -- a negative (ground) output trigger circuit provided by the sensor, usually through which a maximum of 200mA current can flow. If current limitations are met then the circuit can often be protected via an appropriate sized diode, or if current demands are greater than 200mA then a relay must be employed. (I suspect any negative input trigger built into your dashcams would require much less than the maximum current to flow, and as your dashcam would be providing that current, it is completely within your discretion as to how to best incorporate the circuit.)
Besides the negative (ground) output trigger wire, most sensor modules also require a constant 12V positive, and a negative (ground) connection to power their own internal electronics. But the dashcam and hardwire kit would only have to be concerned with the negative (ground) output trigger.
Following are some types of sensors, and the associated installation manuals for reference. Please note that as security sensors are often sold with the intent to be used with specific security systems, the manuals do not always indicate that the sensor trigger output is a negative (ground) pulse, but if you refer to the sections of the manuals in which connecting to third-party alarm systems is described, a negative (ground) output is inferred by the direction of the diode placement in the sample circuit wiring diagrams.
The first type of sensor I would list is a 508D Field Disturbance Sensor. This sensor would allow the dashcam to trigger a recording even if its own internal motion detection algorithm fails to record because its visual field is confused by direct sunlight during the day or insufficient ambient light at night. One could even wire up an auxiliary lighting source to illuminate when the field disturbance sensor is triggered in order to obtain usable video footage during the period of the disturbance. (You can think of this sensor as a configurable invisible radius around your car that when crossed, triggers a saved recording on your camera and can also be used to do whatever else you might be able to dream up too.)
508D Field Disturbance Sensor
These other type of sensors are pretty self explanatory:
504D Stinger Double Guard Shock Sensor
506T Audo Glass Break Sensor
509U Ultrasonic Air Disturbance Sensor
And of course, many security systems themselves can provide a negative (ground) output trigger in response to a button press on the remote control.
Regardless of whichever external sensor(s) are selected by dashcam owners, the dashcam could simply ignore the ground input trigger unless it is currently within the buffered parking mode cycle.
I would suggest that the dashcam include some sort of visual indicator or icon on the recorded video itself to indicate the reason for the buffered parking mode recording -- whether detected movement within the video frame, dashcam internal g-sensor detection, or external triger #1 or #2 if it should exist. That would help owners determine why something was recorded and what to look for in the recording itself if not immediately obvious.
Having an external recording trigger available such as described above would allow integration of the dashcam into more complete security systems -- which is probably why many of us are so interested in the buffered parking mode too.
