How often does this happen?

[PPRuNe Radar]

G-UNIT

If this statement in the BBC piece is correct .....

Because they were critically short of fuel, the (US) aircraft climbed through their cleared flight level, without transponding, entered controlled airspace and conflicted with the Embraer 145

..... then neither TCAS or controllers Conflict Alert tools would work. Both require the aircraft to be transponding so the tools know an aircraft is there and at what altitude.