Human Growth Hormone is a naturally occurring peptide, secreted by the pituitary gland, usually during sleep or after some anabolic training.
If one uses injectable HGH, it rapidly metabolizes into IGF and other subhormones which then assist the body in rapid recovery. It's not an analog/derivative of testosterone which is easy to test for. The HGH spike disappears in the system, within an hour or so. And even its subsequent metabolites don't have lengthy residence periods.
Thus, if one isn't routinely injecting on a regular basis and let's say, juices weekly, chances are, it won't be detected. In addition to injections, an athlete can also shallow 2-3 grams of certain Amino Acids like Arginine/Ornithine/Lysine on an empty stomach daily, and also get a base line IGF increase without it being a case of doping. Taking Amino Acids doesn't violate any rules.