I installed Spotify using the official repository but the icon does not appear in Wingpanel. Someone with the same issue?
Spotify client is a QT-based application which relies on
sni-qt to display it's indicator icon. Maybe somehow
sni-qt got removed by uninstalling another application on your machine.
sni-qt is installed:
dpkg -l | grep sni-qt
In case it's not, reinstall it:
sudo apt-get install sni-qt