Log entry - Class 3 frame received from nonassociated station

Since a few days I noticed numerous of the following events in the System log of my RT-AC68U:
The MAC address is almost always the same, sometimes another and not one I recognize as a known owned device (for new devices I always add their MAC address in a list).
A manufacturer can also not be found for MAC...

Log entry - Class 3 frame received from nonassociated station