Pasir Ris park canal, both sungei api api and sungei tampines, are frequent by otters occasionally. The more regular sightings were dated as early as 2012 and before. In 2014, there was a rare sighting video footage of up to 10 otters at sungei api api. YouTube video can be found here: http://youtu.be/xU7MWcoCpfA
Since early 2015 till date, the big group sighting was hardly seen. The regular otters in Pasir ris are now a pair of mature otters and a young adult. These 2 + 1 otters do not belong to same click, they have not been seen together in 3 at the same time.
The pair is extremely familiar with sungei tampines, they would often swim in from the sea into the small wetland area and then climbed over the rubber dam, into the river in the housing estate. I followed them on quite a few occasions, just to see how far they would go upstream. They actually do not go too far, they will just fish with the range of 500 meters and return back to sea via same route in less than 1 hour each time I saw them.
The lone otter does not go into the housing estate, at least not into upstream of sungei tampines. It often does its round at sungei tampines and sungei api api.
It is quite unlikely their holts are in Pasir ris park itself. The 2 canals are just one of the routes whenever they stop by this area. These 3 otters may have some affiliation to the lor halus family, but this assumption cannot be confirmed further. There had been some sighting reports at changi beach and loyang areas, likely these are the ones.