Sighting of a translucent ball indoors

Location: Zaragoza, Spain. Center of city. Indoors.

Time: sometime along 2000s.

I was with a friend drinking some coffee. We were watching TV. Suddenly, from the direction of the balcony of the apartment, a translucent ball appeared. It moved horizontally. Quite fast and in a matter of 2-3 seconds traversed the room, fussing with the wall. No sound, no erratic movement. It blocked some of the tv image, but distorting it more than completely blocking it. There was no storm. The curtains were fixed so it is almost impossible that it was a reflection from the street outside. It left no mark and dissapeared into the kitchen passing through the wall as if it didn't exist at all.