Hi,
Are there any experience what happens when you fly the spark with no device connected and the low battery rth threshold is reached?

Normally you get a pop up where you can cancel the RtH.

When I start the spark with palm launch, move with tracking a few hundred meters from the HP and battery is almost empty -> what happens?
Will it return to the HP or just land when the critical battery limit is reached?

Please don't guess...

It just lands where it is... Over water or a hole.

Can someone from DJI support verify this? I'm curious too as to what happens as the OP has stated. I hate to be caught out using the palm launch within this scenario.

This actually happened to me - I was recording myself walking through a park in gesture follow and lost track of the battery power - it initiated a low battery RTH whilst in gesture mode and flew itself back to the point where I first took off using palm control and began landing itself. I thought it was a flyaway at first but no, it performed a perfect RTH. I do believe the next FW .700 update will change this to the Sparks last location which will update that new location every 20 seconds so hopefully I won't look like an idiot running after my drone across a field the next time this happens - just be careful using the drone in this mode when near water or near a cliff edge.