un ‧ de ‧ tect ‧ ed /ˌʌndɪˈtektəd◂, ˌʌndɪˈtektɪd◂/ BrE AmE adjective
if something, especially something bad, goes undetected, no one notices it
go/remain undetected
Doctors can make mistakes and diseases can remain undetected.
The thieves escaped undetected through a basement window.