Let me bite, why?
Funnily enough, I gather the authorities missed out on the Plen when he was once right in front of their eyes at the press office
Communist operated their clandestine operations in cells. A typical cell averaged about 5 to 6 members with 3 being the minimum and 7 the maximum. The indoctrination is carried out by a cadre who also leads the cell lasts about 9mths to a year where meetings are fixed and regular usually at a minimum of once a week. On completion, the oath of eternal allegiance is taken.
As a rule, no one says that they are communist or reveal details of fellow cell members of when they became one even when they meet at communist events. If caught, you only can only reveal a maximum of 6 other jokers as that is all you know.
As such, unless you are in the same cell with someone and or was present at the oath taking, you will never be sure who are full fledged communist. So you end up with pro-communist, crypto communist labels.
Intercepted documents such a central committee meeting are the other sure way of identifying communists based on the attendees attending. That only identifies the top but not the rank and file.
The cell leader is usually a die hard steely person that will not breakdown under interrogation so the links between cells or higher up the organisation cannot be ascertained.
Prior to the Japanese Occupation, Special Branch managed to secure the Singapore Town Committee namelist but after that they had not been lucky.
That the primary reason why Chin Peng in his memoirs can gloss over the issues on membership and singapore communist as there were no membership list per se.
The intelligence agencies will usually go by a series of consistent conduct to tag someone and if lucky the confession of a fellow cell member will nail it for sure.
At that time, left wing politics was in vogue in intelligentsia and therefore it became very difficult to make anyone form of accurate identification. john eber, gerald d'cruz, Eu, Sharma, lim Hong Bee etc were known communist because they were very open about it. The bulk of them visiting the queen street CPM office frequently as it was for a short time after the war a legal organisation. Plen revealed himself for the purpose of negotiations and showed his bon fides by removing a communist from workers party.