In Flightradar24 from 290030Z and then for two more hours, there were 4 identical objects over Germany between EDFM and EDDS at altitudes from 600 to 1900 feet, a few knots speed and each changing course more than two times per minute but no trace like if stationary. The distance between the objects was about 20km. They all have the same callsign: "TWR". What are they?
Many towers in Germany are equipped with FLARM so that they are electronically visible to FLARM equipped aircraft in the area. This is especially useful for helicopter emergency medical services and law enforcement that operate in urban areas during marginal weather conditions.
The fact that the towers are not always visible on FR24 may have to do with the fact that not all their receivers are equipped with FLARM. If nobody nearby is providing FLARM data to FR24, the towers are not visible on FR24.
The speed shown has to do with the GPS receiver in the FLARM unit; they sometimes detect small speeds, even though the receiver is stationary.