I would be very much interested in reading more about this, do you have any good articles you could point me to?
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2657495/
Paper on challenges of detecting growth hormone
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3830297/
Testing for both of those substances has improved, but still leave plenty of room for abuse.
Use of "unknown" or the constant stream newly developed substances is naturally not rigorously studied, but here's a couple of articles referencing the challenges they pose.
https://thetruthaboutforensicscience.com/undetectable-perfor...
https://bleacherreport.com/articles/1955231-undetectable-the...
UFC is notorious for testosterone TUEs, but that's mostly because they regulate things differently to most other sports
https://www.espn.com/espn/otl/story/_/id/10500652/therapeuti...
Top flight soccer is another sport that's constantly garnering attention for TUEs, but most of the coverage of that is speculative, because other than high level statistics the TUE process is naturally not very transparent (and most of the recent coverage on the topic has been replaced by controversy relating to trans athletes).