For the last week or so my 5-year old has suddenly started soiling herself. has also started hiding soiled panties so believe it is not deliberate or attention-seeking. please advise.

Possibilities: This is often a feature found in kids with the problem called stool retention.They had pain on pooping at some point with a hard or bulky stool. They try to avoid going and weaken the anus over time, and begin leaking when they no longer feel it. I would seek out a pedi or pedi gastroenterologist with experience dealing with soilers.It can be fixed but often takes up to a year of hard work.