Water form the wash basin trap sometimes appears in the bowl in the basement flat of a high rise building What is the most likely cause induced siphon-age wavering out self siphon-age?

Answer 1

There is a strong change that the sewage pipework at ground level may be starting to have issue of clogging up.. Water will always find its natural level. If the waste for the basement is lower than the main sewage line outside the building it will come back.

Answer 2

wavering out