What do multiple slashes in METAR mean? The current METAR for Stanstead in the UK says:


  EGSS 031520Z AUTO 29011KT 9999 FEW021/// //////TCU 07/02 Q1022


I do not know how to interpret the /// and //////. From what I remember multiple slashes means that data is missing, however the data in this metar seems complete. 


What does FEW021/// mean?
TCU means Towering Cumulous, which makes sense, but what do the leading ////// mean in this context?
Complete, unless you want to know the height of base of those TCU's!

The bulk of the information you're after can be found in AIS-GEN 3.5 Section 10.17 - AUTO METAR coding

In describing the coding used by automated systems, it gives this example


  NNNhhh/// Cloud detected, but it is unknown whether it is a convective cloud type (ie TCU or CB)


And this one


  //////TCU Towering Cumulus detected, but no information on its height or cloud cover is available


Which more-or-less covers your two questions, I hope.