This bird looks like it might be the white-rumped munia (Lonchura striata), known in Sinhala as Nitamba Sudu Vikurulla.
The adults tend to have a blacker head and throat, while juveniles are browner, so it seems to fit.
I only noticed this bird hanging around the last few weeks or so, and then two of them started hanging around, and then one of them started working on building a nest up on top of a rainwater drain pipe between the eavestroughing and the roof in a nice little corner.
And now I haven’t seen it for a couple of days again.
I hope they come back and use the nest they’ve been building. 🙂