Hawkeye80: Went back this evening and conducted a wider search of the area.
A true mugglling with no trace of log book, container, or even the piece of wood which was on top of it, securing/hiding it.
I'm not intending to replace in this area, as it's obviously frequented by the wrong type of person.
I will also ensure that any caches I put out in the future will be 'members only', as it must surely be more than coincidence that it went missing in the same week that I took off the 'Members Only' marker & opened it up to everyone.
Thanks for the great comments from all those who managed to find it before it was stolen.