This does seem really odd, for a variety of reasons :
First, since attributes do tend to be higher than skills, this approach is very likely to give characters better chances at noticing something when they are NOT actively looking for it – which is quite illogical.
Secondly, it seems somewhat paradoxical that a character’s Ingenuity should help him noticing things when he is NOT applying his mind to the task, especially since Ingenuity has no incidence on active perception. So, basically, you do not need Ingenuity to notice things when you ARE looking for them – but you do need it when you are NOT… No matter how hard you try to rationalize this, this really makes no sense.
The matter is further complicated by the lack of a general “detection” skill – sure, there are various situations where Survival, Subterfuge or Technology can work well as detection skills but in many cases, the GM will find it difficult to associate a specific skill to the simple task of looking for something odd.
The following option can be used to solve these three problems – the “better passive than active” problem, the “Ingenuity paradox” and the lack of a general, all-purpose perception skill.
For active perception, use Awareness+Ingenuity. In situations where a specific skill (such as Survival or Subterfuge) may apply, characters may use this skill instead of their Ingenuity – but only if this gives them better chances.
For passive perception, simply use the character’s raw Awareness, without adding any skill or second attribute. Sure, this will make things quite difficult – but hey, that’s all passive perception should be all about – raw Awareness, no active application of Ingenuity or skill and far smaller chances to notice things than if you were actively trying. Note that this also makes Keen Senses an even more crucial asset in passive perception situations.