The scene....pressure fairly low (990) and the initial clearance was to climb to 3000 feet on the qnh 990. The aircraft appears on radar passing 3300 feet(mode charlie as yet unverified)reaches 3400 feet then descends to 3100 feet whence the aircraft checks in at 3000 feet. Atco .(me)verifies mode c and with the aircraft identified instructs climb to FL090. In a quiet moment with all traffic off the frequency I ask the crew about the level "excursion".I am informed that they were level at three thousand feet but that because of a high energy departure the mode charlie has spiked. In 28 years I have never heard of this. I am concerned because if this is not a spoof then the potential for TCAS TA/RA is large. What do you think? Spoofing(and treating me like an idiot
) or a genuine phenomenon?