Left 4 Dead 2

Left 4 Dead 2

RMS Titanic (v2.0.1)
Pudgemountain 26. mar. 2021 kl. 9:49
Understanding the Pianos.
I played this map many times and noticed there are arrows pointing at Pianos and when you press E they play or in one case IE Danger Piano it plays and spawns a tank. But is there a easter egg if you play all the pianos or are they just for fun?

If the Former is the case then where are the locations I might be missing one or two.