BERLIN (AP) — Police in Munich are investigating a mystery: More than 1,000 stickers were put on gravestones and wooden crosses at three cemeteries in the German city, without any indication of where they came from or why.

The 5x3.5-centimeter (1.95x1.2-inch) stickers are printed with a QR code, that, when scanned, shows the name of the person buried in the grave and its location in the cemetery — but nothing else.

“We haven’t found any pattern behind this yet. The stickers were put both on decades-old gravestones and very new graves that so far only have a wooden cross,” police spokesperson Christian Drexler told The Associated Press on Wednesday.

“People who have witnessed anybody putting the stickers on the graves are asked to reach out to the respective cemetery’s administration,” Drexler said.

The stickers surfaced in recent days at the Waldfriedhof, Sendlinger Friedhof and Friedhof Solln cemeteries.

Police are not only trying to find out who is behind the stickers, but are also investigating property damage, because the gravestones were partially damaged and discolored when the stickers were removed.