Aren't sensors designed to detect everything?
Well, no - just like phasers aren't designed to destroy everything, shields aren't designed to protect from everything, and warp engines aren't designed to provide all speeds from zero to infinite.
(Actually, that last one is debatable, as it does appear as if a warp engine
can attain any and every speed if enough power is somehow fed into it...)
Sure, the designers would like to include everything. But there's no way they can. And apparently, theta flux distortions are just a tad too difficult to detect.
"The sensors were never designed to detect such distortions, yet there was a record of the distortion in the sensor log." How is that possible?
The sensors evolved, on their own. That was the point of the whole episode.
I guess there would exist sensors that can detect theta radiation, and analysis systems that can measure the flux involved, but there just didn't used to be enough computing oomph to divine any distortions from that feed - a bit like how a gamma-ray telescope today wouldn't have the same sort of resolution that, say, an infrared telescope has, due to fundamental physical issues, and would have to do a lot more computing to compensate. But the computer evolved, reallocated, and ultimately survived thanks to this bit of evolution (which in the case of intelligences needn't be Darwinian and can quite blatantly be Lamarckian - and can take place in milliseconds, meaning a bit of idle experimentation on whether theta could be resolved better can alert the computer to something being amiss with theta, at which point it chooses to evolve to meet the potential threat).
Timo Saloniemi