IO540:
I was referring to the situation where an aircraft outside controlled airspace receiving a RAS comes into conflict with a no-mode-C unknown aircraft - happened to me again this morning, and a passenger carrying flight had to be vectored all over the sky to avoid a 7000 squawk that was probably no threat whatsoever
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With aircraft inside controlled airspace, like airways, it is assumed that any unknown traffic is outside controlled airspace unless the controller has reason (eg., mode 'C' returns) to believe it is inside controlled airspace - lost etc. This brings up another situation which has been mentioned - if you inadvertantly strayed into an airway or control zone without mode C selected. The controller would assume you were beneath the airway or above the control zone unless they had any other reason to suggest you weren't, and would ignore you. In these circumstances, quite apart from the ramifications of the airspace infringement, leaving mode C on is a good way to avoid an airprox or worse
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