In motion detect mode, dashcams are already on, it is not like they have a PIR sensor that detect movement.
If a camera was off or just in a stand by mode you would loose the actual triggering event as the camera need to wake up and get going first.
So the cameras are already on and constantly buffering, so if there is a trigger you will also get a few seconds from before that ( at least good dashcams have that )
But in general we advise against using motion detect as it is not ideal for a couple of reasons.
1: Most often motion detect are too sensitive, even it set to the lowest setting, so just about any movement will trigger a recording, and so if you park anywhere with movement you might as well just record all the time.
It have been this way for a long long time with dashcams, and at least to me it is a bit strange they have not dialed this in yet, but okay i tend to over simplify things.
2: there have also been a few cases of dashcams bot being sensitive enough, but these are very rare, by far the general problem are too sensitive.
Other ways:
You also have the G-sensor trigger mechanism, but it only trigger when your car have actually been hit, but otherwise you can also have buffering on this so you still get a few seconds leading up to the event.
Again here the issue are of sensitivity, but the few times i have tested it, it have worked okay, last time i was on the B1W camera and just kicking the rear wheel triggered a event, and you cant kick a steel wheel that hard wearing sneakers.
But now that i have been given the chance to test parking mode with a proper hard wire kit and system with the Street Guardian SG9663DR camera i was sent i am instead using the always on recording but with a low bitrate to conserve memory space.
This model record 20 mbit in normal mode but the parking mode are just 5 mbit, which are sufficient when things are not moving fast, and another + is that the recordings also have sound.
Newer cameras also have the option to do time lapse parking mode, this also run all the time but no sound and time lapse with either 1-2-5-10 frames a second, this also conserve memory card space, and dont generate as much heat in the camera as full bore recording, and heat are a issue when the car is parked on a hot day,,,,,, it can even get too hot and the camera must shut down to protect itself.
This video demo how the DR system cope with parking mode next to a road where cars go by at some speed.
In parking mode the 3 minute video segment size i use are just 120 MB, the full blown recordings are 4 X larger
Parking in general:
To do parking mode you most often need a hard wire kit, most often with newer cameras that kit are a 3 wire kit, meaning you have connection to a always on +12 V circuit and a +12 V ACC circuit and then of course a ground connection.
In the parking mode kits you will most often find low voltage protection, this mean you can set a minimum voltage that when reached will turn off the camera, this voltage should not be set to lower then 12.2 V, but can often be set as low as 11.8 V which is draining the battery far too much.
Some hard wire kits can still be 2 wire, in which case instead of the ACC wire telling the camera what mode to go into, the G-sensor in the camera will tell it to switch to parking mode after a few minutes of no movement.
These are often 12 V cameras and in that case the low voltage cut off might be in the camera itself.
But the majority of cameras today are 5 Volts, so the hard wire kit will also have a converter build in changing 12 V to the 5 V the camera need, 12 V cameras use a barrel style plug in the camera, 5 V cameras most often use USB micro / mini and USB C are also in a few cameras now.
Most dashcams today require use of their own hard wire kit, so the old generic hard wire kits can not be used,,,,,,, these cheap generic kits can also be pretty bad so people should avoid those.
Parking mode peril's :
Newer cars can be too smart some times, and can do all kind of funny stuff when detecting a parasitic drain like a dashcam in parking mode are, they can also keep circuits on for a while after parked, so finding the right fuses to tap into can be a pain.
Additional INFO: as i dont really need parking mode, the Dr model i was sent to testing also have a timer, and i use that and have set it to just 1 hour which cover all shopping i do, and should i need more i can just up the timer all the way to 24 hours or when my 2012 battery in my small car drop below the cut off voltage.
Also using parking mode, well it use power so you have to drive some to offset that drain, so you can not use parking guard for 23 hours a day and only drive 1 hour, even though a car battery get charged with a lot of Amps when the car is running.
Most dashcams allow you for shutting off the camera, say if you park in a garage at home, where parking guard would just be wasted, most often you do this by long pressing one of the buttons.
Next day the camera will start recording when you start the car.
if you park in your driveway you are better off having CCTV on the car, i have this ( use to have camera broke ) on my car parked in the back yard, with a camera mounted on my 2 floor balcony door.
This was a must as the back yard are completely black at night, so need a IR light aided camera.
