The Guangzhou Modern History Museum apparently used to be a police headquarters, which is hard to believe given the exterior. It's nestled in a corner of the Martyr's Park area which was extensively upgraded (for the surrounding landscaping) in 2010. It is integrated into the Martyr's Park complex.
The stone bridge approaching the museum and the lake and generally idyllic surrounds make for popular areas for photos and selfies especially on the weekend, and most people in the area are just enjoying the surrounds rather than entering the museum.
There's a lot to see in the museum, though not much in English. The ground floor covers the history of Guangzhou from roughly the first Opium War in 1839 through to the formation of the PRC in 1949, with extensive attention to the various Guangzhou uprisings. There is a rich photographic record but no English. On the second floor the emphasis changes from politics to economics and everyday life.
The cache can be reached from either side of the fence