imperceptus (inp-), a, um, adj. [2. in - perceptus],
unperceived , unknown (very rare): mendacia, Ov. M. 9, 711: minora majoribus imperceptiora sunt, more incomprehensible , Gell. 14, 1, 24.
imperceptus (in-p-) adj., unperceived, undetected: mendacia, O. ...
An Elementary Latin Dictionary