Hi Dave
I get a 000000 almost all the time here in Chorley, sometimes with valid callsigns like LEUCHA20, GETSUM11, EPS238, DLH014, GGFEY etc, but most often without any callsign or other data.
I've got it at the moment, squawking 0254 but no other info at all. It's so common that I hardly notice it.
I put most of it down to BAE Warton, which is in direct line of sight, where mysterious things may be happening. But I'm sure that some of them are genuinely broken or miscoded transponders.
Rod