It seems that this year, both Hong Kong and Macau decided to stop stamping visitors' passports upon entry to and exit from the country. Instead, visitors receive a slip of paper with pertinent details of their entry, stapled or slipped into their passports.
Why is this important? Some punters come to Hong Kong, and would like to go to Macau for an evening or weekend, but don't want to return home with a "Macau" stamp in their passports. Now, this is no longer a problem. The slips of paper can simply be removed, leaving no evidence of the visit in the passport.
My most recent visit to both Hong Kong and Macau worked exactly this way, so I can confirm it is true.
Happy punting!
Gherk