She-

... Yeah, Ray, sure. She cloned herself. Had no idea she knew how to do that until this moment.

[Hell. Venkman pulls out Kirby's containment unit and lets her out- might as well just make this a little family reunion. Kirby hisses immediately at the sight of Egon, but within seconds her demeanor changes. She starts chattering excitedly, waving her shadow-y tendrils and reaching for her children.]

I dunno. Maybe since they were so close to hatching, the meter picked up on them?