That was something I was curious about. If it leaves swathes of destruction and eats everything, even if it wasn't constantly moving to search for more food, it'd be so obvious, and since it's nearly a godzilla level threat (one in a city could honestly knock over buildings) they'd have KE or LS or whomever on it to kill it immediately. Unless Shadowrun PETA is fighting for them not to be eradicated and somehow winning, then they'd be killed on sight and they're NOT subtle, so unless the pop out of the ground fully grown, birthed from the core of the planet, I don't see how they can just "show up".
Like, they say nobody's gotten a chance to study one, but there's no given reason why tracking chips wouldn't work. Even assuming they don't know if such things have a nest or how they have babies or whatever, if there's one, you'd look for the rest and destroy them. Maybe that'd be the bounty? "A juggernaut was killed in the area recently, a bounty will be given to anyone who happens to eliminate any offspring if found."
The "how did it get here without being found when it's MO is a swath of destruction" thing bothers me the most.