Magnetic Field of Toroidal Transformers
It very much depends on the transformer (and how close to saturation the thing is being run), a good quality toroid run well within its primary voltage rating will have surprisingly low leakage, while a cheap one run on the ragged edge of saturation (Say because it is a minimum cost 60Hz part being run on 50Hz without appropriate derating) will spray (harmonically rich) fields around. 

For shielding, flux bands are good, and actually once you are a few inches away, ordinary mild steel is surprisingly effective (And cheap), mu metal is expensive and saturates easily, it is more applicable to small fields then mains transformers of any size. 

One other thing that is something of a surprise is that rotating the toroid on its mounting bolt can often find an angle where the field at whatever device matters is largely cancelled.

The valve audio guys tend to be the people who really have to deal with this crap.

I would BTW be highly sceptical of that meter, direction matters and anything which just has a 'microwave' range without a table of correction factors has me dubious.