it depends on your building. A door spy cam is easiest. The Cams need wiring. Please are plenty of small and cheap cams out there.
The HDD VCRs have usually a video motion trigger, so you get only something recorded when something moves.
WECL in SSP has a very good cam and recorder selection.
WECL Online
I had a similar experience with a contractor before! Seems like this is a common problem in Hong Kong even for locals. I've heard many of the contractors failed to complete their work and would simply disappear and turn off their phones. My friend tried to report it to the police too and was told the same thing. Luckily I found a reputable and english-speaking contractor in the end. He's not cheap but his work was of high quality and he's easy to communicate with.
Good luck ccyuen!
Interesting because I just had some work done in the states. I simply called for a quotation and it was a big job. Entire sewer line replacement. We told him we wanted it done then never heard from him. I was calling and setting up other appointments too. Then suddenly he called and said it's done. This is like a 2-3 day job plus need inspection by the city. Then he called because he wanted to get paid. I was sceptical at first (living in Hk too long, lol) so I asked for photos of the work and I checked the city inspection report. Then paid him. So they did the work first then asked to be paid after.
The thought did cross my mind and ways to ask for further discount. Lol.
For the time being, they have been pretty quiet; but I just found out that his company is not certified to carry out electrical works, after checking with the Electricity Legislation Division. The ELD now want to pursue the matter and charge him, but I am worried that the contractor will know its me and come after me using triads, any view to help me make a decision on whether to step up allow ELD to state my name and make use of the quotation.
@ccyuen - often contractors will quote you, but sub-contract the work to a licensed person. Are you sure that the person who actually did the work is not licensed?