Some says at punggol/sengkang areas where all those 'people' used to roam as there are old trees/jungles. Once cleared, those 'folks' got nowhere to go and sometimes have hang around the HDB estates. If anybody knock on the door just after sunset, better dun open. If you answer the door, they will want to go into the house in pretext that they want to use the toilet, etc. Once in, hard to let them out...