@Wayne-TP 

Thanks for the response.

I've been through the linked article and the camera seems to be set up corectly. I also used the detection range test, and the camera kept flashing white lights at me while I moved in the detection area, so that part of it seems to work okay (although I did this during daylight; our interloper was there at night).

Although battery-operated, the camera has a solar panel attached and as a result the battery power is typically very high - it's currently at 100%.

The camera uses cloud storage via a Tapo hub in the house.

My main question, though, is why did it record for just one second? Notwithstanding the fact that he'd clearly been there for a while before the recording started, why did it not continue to record after it had identified him? I actually have the record buffer at nine seconds, so even if it had stopped recognising that there was someone there, it should have continued to record for a further nine seconds.

@DuncanKent I've previously suggested the record buffer could have longer options as I also miss recordings. 
Please give it a 'vote' if this is something you would like: https://community.tp-link.com/en/smart-home/forum/topic/731638

Here's a related request from another customer which would help people capture things by using a movement sensor and would be great to have:
Trigger C420 camera recording smart action when T100 sensor detects motion