House unoccupied?
Let's look at it slightly differently.
Plumber goes in, drains down (did he drain the system or just the roads he took off?).
Let's say he drained down.
Took of x number of rads, didn't cap valves as system drained.
Yes filling loop still attached, yet who can say that they don't see this more than 95% of the time. Plus is it a separate filling loop or built into boiler like a worcester?
Third party working in property, ooh it's a bit nippy in here, I will shot the heating on. Oops, no pressure, never mind it's just like mine at home, I'll stick some water in it.
Splash.
Just trying to play devils advocate and always 2 sides to a story.