Formal:
* Overlooked
* Unremarked
* Unperceived
* Unnoticed
* Missed
* Ignored
Informal:
* Missed
* Missed the mark
* Missed the boat
* Missed the memo
* Blind to
* Didn't catch
More specific:
* Hidden in plain sight: For something that was visible but not consciously noticed.
* Under the radar: For something that went unnoticed because it was not considered important.
* Passed over: For something that was ignored or not given attention.
* Slipped through the cracks: For something that was overlooked due to a lack of attention or organization.
The best synonym will depend on the specific situation and the tone you want to convey.