IPA: /ˌʌn.dɪˈtɛk.tɪd/
KK: /ʌnˈdɛtɛktɪd/
Not discovered or noticed; something that remains hidden or unobserved.
The thief left the scene undetected, avoiding all security cameras.
Undetected → It is formed from "un-" (meaning not) and "detected" (from Latin "detegere", meaning to uncover or reveal). The word "undetected" means not uncovered or not revealed, indicating something that has gone unnoticed.
Think of something that is 'not' ('un-') 'revealed' ('detected') — that's why undetected means it hasn't been noticed.