2c389a02__Specifications___openEHR_wiki__openEHR_equivalent
[0] 3 [Code mnemonic] TRC [Rubric] trace [Description] The content is greater than zero, but too small to be quantified. [openEHR equivalent] 
This is a laboratory concept, and has nothing to do with missing data; labs routinely specify some amounts as 'trace'.The value not in itself computable, but is usually part of an ordered set, e.g. trace, +, ++ etc. Usually modelled in openEHR using DV_ORDINALs, e.g. for urinalysis. Yes, no, maybe. In the lab, you may report a value, or TRC. TRC is used outside ordinals - though not as often, I agree
Null Flavours and Boolean data in openEHR - Specifications - openEHR wiki
openEHR equivalent