Yup, I think many people assume so, and they may be assuming wrongly.
Nowadays manufacturers are wising up to this "unfilled warranty - free extension" idea. I know that Nikon has been requesting for proof of purchase for quite a number of years now, and even if you lose your receipt, they can track down its purchase place and date.
Sony has even gone a step further by having an "expiry date" on the warranty card! So therefore, people who sell goods on the 2nd hand market indicating an "unfilled warranty card" may not be doing anybody any real favours.