They probably got tipped off by something else that triggered a deeper investigation. It is not economical to do x-ray spectrometry on every single machine part leaving HK.
\0 Social tipoff - did somebody snitch in advance?
\1 Behavioural - is the sender a regular dealer in machine goods | compressors | etc .. a financial accountant suddenly transfering compressors is a behavoural spike.
\2 Consistencey WRT thousands of items per day ... does this load weigh the same as other loads on a pallet with the same Bill of Loading?
\3 Actual Tomography - it's easier than many might think to calculate a rough volume at speed via either X-Ray and|or paired oblique laser scans .. combine that with conveyer weight sensor and you have a density ... see \2.
All these things raise flags .. depending on the number and types of flags per hour, things may or may not get a closer look.
Come to think of it, airport luggage inspection happens at high throughput. Surely they could scale that up from suitcases to containers?
Of course, "physically possible" doesn't mean "politically possible given the budgets involved" which in turn doesn't mean "has actually happened." I'm mostly fishing here to see if anybody knows things about high-throughput X-Ray screening.
I mean by that logic you could also have a couple of mules wear expensive jewelry or watches or the like; if it's convincing enough that someone wears a $500K watch as personal effect then they wouldn't be flagged for inspection.
In theory anyway, I don't know how these things work in reality. I wonder if someone's financial status is pulled up at passport control.
I don't believe it's uncommon for rich people to travel coach though.