A word for something that used to be unique but is now so commonplace it is no longer noticed

You are probably thinking of the word ubiquitous.

: existing or being everywhere at the same time
_{Merriam-Webster}

However, this is usually used to connote "can be found anywhere" rather than "so common, it's presence is unnoticed". Another -ous word that does connote being unnoticed would be inconspicuous, but it lacks the connotation of "being everywhere".

: not readily noticeable
_{Merriam-Webster}

So, your sample sentence could be completed with inconspicuously ubiquitous.

Giving the used to be unique constraint more consideration (prompted by Edwin's comment), yet another -ous word occurred to me: insidious. For the specific example of metal detectors becoming ubiquitous, you can argue the situation should be viewed in a negative light. Insidious imparts the notion of a bad situation gradually becoming more pervasive. However, it also gives the connotation of becoming more noticeable over time, not less.

1 a : awaiting a chance to entrap : treacherous
1 b : harmful but enticing : seductive <insidious drugs>
2 a : having a gradual and cumulative effect : subtle <the insidious pressures of modern life>
2 b of a disease : developing so gradually as to be well established before becoming apparent
_{Merriam-Webster}

So, another way to end your sample sentence would be insidiously inconspicuous.

The sample sentence could be rephrased

When metal detectors become the norm rather than the exception, we will no longer notice them.

The OP is describing a situation where something that was once an "exception" is now accepted as being "unremarkable" and standard.

norm
something that is usual, typical, or standard.
_{(Oxford Living Dictionaries)}

You could also consider a phrase rather than a word.

the new normal

a previously unfamiliar or atypical situation that has become standard, usual, or expected.

Your example sentence would then become

When metal detectors in malls become the new normal ...

The use of the phrase "new normal" clearly implies that their ubiquity is new.

How about ubiquitous?

Present, appearing, or found everywhere:
‘his ubiquitous influence was felt by all the family’
‘cowboy hats are ubiquitous among the male singers’