I was flying yesterday and detecting an aircraft showing only its HEX code and not registration (I use SE2 for ADS-B In/FLARM reception).

When you say showing, where did you see this?

HexID only is highly probable a 868MHz uncertified unregulated sender. These become increasingly annoying and don't even follow the aircraft ID regulations. 868MHz or other Flarm frequencies are completely let loose and many tinker with homebuilt devices, totally legal. Get used to it for the time being, Be lucky if it was not the garage door opener or home automation device down below, they use the same bands and btw some companies already began using the frequencies for other tracking operations - nothing to prevent that.

I saw the HEX code only display on my SD moving map display.

It's possible the traffic received was from a Flarm device but was broadcasting an ADS-B hex code. In that case, we would not have the registration to give you, so we would display the hex code instead.