You are probably thinking of the word ubiquitous.
: existing or being everywhere at the same time
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
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
So, another way to end your sample sentence would be insidiously inconspicuous.