In this age of mobile phone cameras everywhere, we should not be surprised that footage of her surrounded by police before her death, in distress, has been found:

https://www.reddit.com/r/HongKong/comments/di5kjy/chan_yin_lam_the_15yearold_competitive_swimmer/

From Reddit user /u/lebbe:

According to the narrator the man in striped shirt was a plainclothes officer with warrant card.

In this video Chan was wearing the same top she did in her IG video

This was supposed to be in Aug 12.

So far her timeline seems to be:

Aug 11: she was out protesting. Made an IG video

Aug 12: seen in this video in distraught while being surrounded by police. Wearing the same top she did in her IG video

Sept 12: was seen again after disappearing for 1 month. She said she was in juvenile center, which had become the favorite method of HK police to lock up underaged protesters without trial

Sept 19: last time seen alive

Sept 20: her mom came back from travel. started looking for Chan

Sept 21: mom put up posters and facebook posts looking for Chan

Sept 22: body found floating in water

Sept 26: Chan's family notified by police. police ruled death suicide, no foul play. Family asked for a second autopsy

Sept 27: second autopsy

Oct 10: body cremated without going through regular scheduling

Oct 11: press was notified of Chan's death

Source