First thing came to mind is that maybe cortexifan kids can travel without repercussions not only for themselves, but for universes' fabric too, so they could go undetected. Or maybe it's Olivia's unique ability.
That raises the other questions, such as:
- Were Walter and cortexifan kids detected only because they have taken Walter with them?
- Does the shape shifter crossing over create a breach? (probably not, but was it said anywhere on screen?)
- How did Peter coming back to his native universe affect their breach detecting devices? (Even if he himself is from here, his clothes and other possessions are from BLUE universe)
- Or does breach occure only if alive or souled matter [being?] is crossing over? (If only alive things, Peter would still be detected, as we all have simbiotic bacteria on and in us - and his still would be from BLUE universe )
- If the answer to (4) and shape shifter question(2) is yes, does it mean that shape shifters aren't alive, or does it prove them to be soulless?
- Maybe breach isn't connected to crossing itself, as Joshua Rose was successfully imitating devices’ reaction by destroying the fabric of universe from inside (he wasn’t crossing to our universe, was he? Or maybe he was putting part of our universe inside his ring)?
- Can it be that breach related to crossing over is anisotropic, thus detected only when crossing from BLUE to RED universe happen? And if crossing is from RED to BLUE, nothing breach-worthy occured? This would mean that all deteoration of RED universe is Walter's fault (or somebody's, but definitely from our universe).