Here, HY analyses cherry picking and mistranslations from Timothy Grose (Twitter thread).

Timothy Grose presents several images intended to show the existence of detention centres. This is directed at Roderic Day.

Note the 看守所 on the sign behind the officers, highlighted by HY.

HY comments: “Further down in the Dec thread, there is a reference to terminology incl. 看守所. When looking at the photos Grose posted, I had to do a double take, because the separate term 监管支队 is mentioned first, and only in a storified passage later is 看守所 mentioned.”

(For English speakers: 监管支队 means “supervision detachment”, 看守所 means 'detention centre')

As HY points out, “First, what is the relationship between 监管支队 and 看守所? This isn't clear-cut, one Zhihu says says it can be assigned to BOTH 拘留所 and 看守所; this edu site says it cannot. (拘留所 = lockup)“
“Regardless, what is the purpose of 看守所? No it is NOT exterior to legal frameworks ('extrajudicial', as someone in the quoted Dec thread seemed to imply), but it CAN hold suspects for far longer than 15 days.”