"nearly undetectable" is correct and usable in written English, and can be used in any formal or informal context.
Example: The noise produced by the fan was nearly undetectable.
The cave in back of General Hafiz's command post is nearly undetectable from a distance.
Although she has a nearly undetectable viral load and no opportunistic infections, her doctors at U.C.S.F.
Neutrino, a nearly undetectable particle with a capacity for constant movement, came immediately to mind.
Nearly undetectable haemolysis is observed at the MIC.
The real value of this system is that the camera is nearly undetectable.
Differences in the physical composition of the stones are nearly undetectable.
Yet WIMPs would be nearly undetectable otherwise, as they would interact with ordinary atoms so feebly.
Hawking radiation from black holes would be so weak as to be nearly undetectable, however.
Interestingly, COMP remained nearly undetectable in normally healing wounds where myofibroblasts transiently accumulate in the granulation tissue.
Relative abundance of Aeromonadaceae was ~5 % during day 1 to 116, increased to 32.7 % by day 130, and declined to nearly undetectable level thereafter.
Stability in the adsorbed phase increased dramatically with increasing loading, and unfolding was nearly undetectable close to the resin saturation capacity.
