The "Piano Moment" was part of the 50th Anniversary of ABBA's Eurovision win.

Benny pre-recorded a short piano piece of Waterloo and in four European capitals, namely London, Berlin, Warsaw and Stockholm, lone pianos were on display which automatically played the piece as if Benny was there in person.

In England the piano was on the concourse at Waterloo Station.
In Stockholm, it was at the department store, NK.
In Warsaw, it was at chopin Airport and
In Berlin, it was at the foyer of Friedrichstadt-Palast.

A video was released a few days afterwards showing highlights of the four locations and also Benny starting and finishing the piano piece watched by Björn.

YouTube link c/o ABBA.