I have been pondering this as well. Is detained an action or condition (estar) or a description (ser)?
Looking at my SER/ESTAR notes
PLACE...P/position L/location A/action C/condition E/emotion
SER DOCTOR D/description O/occupation C/characteristic T/time O/origin R/relationship
Exception Note: 'location' of 'events' is SER
Note: the following examples are conditions and not descriptions Sick/dead.tired/happy etc. These use ESTAR
I am sure there are many I have not learned but this is a helping tool