Thank you everyone. I am not sure why sensors drop off the network.
It doesn't happen very often, probably one sensor in two or three weeks. Not frequent enough for me to investigate.
Usually it is the same motion sensor, which would probably be a location issue. However this sensor is not at the edge of the network, there are sensor further than this one but continue to function perfectly.
As for the xiaomi sensor, it dropped off yesterday for the first time in 3 months. Again, not sure why.
I have couple of Nue smart outlet around the house which suppose to act as repeater, but it is not proven how effective they are.
I am waiting for ikea repeater to be available here in Australia and I will get a bunch of them around the house, hopefully that will strengthen the mesh and get rid of this issue