Hi Jaap,
I suspect, but don't have much in the way of evidence, that after mid-March 1945, NJG2 maintained detachments of Ju88s at Steenwijk-Havelte and Leeuwarden, obstensibly for Fernnachtjagd, and, certainly, Ju88s of 8./NJG2 did fly Nachtschlacht operations from Leeuwarden in late March 1945. Four Ju88s were supposed to take off from Steenwijk-Havelte for Fernnachtjagd on the night of 17th-18th March 1945, according to a warning message broadcast for the benefit of Flak units, and intercepted by the British "Y" organisation. By this date, only NJG2 was tasked with Fernnachtjagd.
II./NJG4 had a detachment of 7 aircraft at Steenwijk-Havelte in March 1945, but all but one of these aircraft had been transferred back to Germany by the 13th March. The remaining unserviceable aircraft was not 501, but a G1. This G1 - 714517 of 6./NJG4 - went off strength with II./NJG4 on 20th March due to enemy action, but it is not clear if the aircraft was still at Steenwijk-Havelte at that time.
While not directly answering your question concerning 501, I hope it may offer potential pointers in the right direction
Cheers
rod