My probe ground has always been the TV's chassis which I've always been concerned about since it always has something like 60v running through it regardless of the set being on or off. I've tested everything on the primary side. Even disconnected everything form the terminals of the plug on the back. It's almost like the socket itself has a leak but if you check there's no resistance under 20 megaohms.
It's always entirely possible the bekelite insulator on the socket has broken down with time but god knows what adding 60v AC to the chassis has done.