Due to high rentals in Hong Kong, there have been more and more upstairs shops in recent years. These are stores in mostly nondescript commercial buildings and may not have a street level signboard. The upstairs shops may not be limited to retail and restaurants are common (this article features a selection of upstairs restaurants, bars and art galleries).
they're not on strike, just holding advertising placards
The upstairs shops typically advertise by distributing flyers and people holding advertising placards in busy areas.
Sample flyer with shop outlet address
I have found pretty good deals at some of these shops before but it is very often hit and miss. Not all stores sell genuine products, so you do have to know what to look out for. Some of them are actually very good copies (typically for bags etc). The beauty upstairs shops I've personally been to sells nail polishes, Hong Kong domestic brands and Korean skincare brands (not available outside Korea).
These shops are usually at non-touristy buildings, so if you're interested in the brands after taking the flyers, it's best to ask the person distributing the flyer where exactly the location is. A Hong Kong acquaintance I was speaking told me that some upstairs shops may only stay in a certain location/space for 6 months, so if you had fun shopping at any particular store, it's best to get their Facebook page or website so that you can be informed of any move.
Due to the space contraints, the upstairs retail stores tend to be bare bones - think simply painted walls, movable shelves and display counters and a cash register. Some may only accept cash in Hong Kong dollar or their local bank network cards (EPS), so it's best to prepare Hong Kong dollars before shopping there.
I have more pictures and details of one such upstairs shop (Cher2) here.