They'll Never Know
Pinkie and secrecy are like fire and methane. Very occasionally they will be observed together in the same room, but only for a short period of time, and it'll all end in a fiery ball of death anyway.
Likewise, Rainbow Dash is no shrinking violet. A life of thrill-seeking and attention grabbing prove that.
But this time, they need secrecy. They need time, and silence. They need to figure it out, this thing they've got, before it goes or they get hurt. It's too precious to lose just because the world's watching over their haunches.
Until they know, nopony does.