I just saw this (see attachment) out of my living room window. I guess they turned around at about 11:00 CET (GMT+2) today, 23-SEP-21. I could not find it in AR view of FR24. He was heading somewhere around 270 before the turn. And probably about 10 miles East of Antwerp (EBAW).
It would be interesting to find out who it was and why they turned around like that.
Pretty likely a military fighter jet with transponder turned off.
Flightaware and Flightradar are also filtering this normally. You might find such aircraft only on ADSBExchange (in case the transponder is active)