start xterm
run cat
press AUX

=> ^@ is printed

xev shows

KeyPress event, serial 26, synthetic NO, window 0x1200001,
    root 0x3b, subw 0x0, time 687714203, (352,234), root:(352,234),
    state 0x0, keycode 177 (keysym 0x0, NoSymbol), same_screen YES,
    XLookupString gives 1 bytes: (00) ""
    XmbLookupString gives 1 bytes: (00) ""
    XFilterEvent returns: False

xmodmap -pke shows "keycode 177 = "

what causes the ^@ to be printed?