Looking through Scan, isn't it made obvious(ish) in the warranty section of each product? If it says "Manufacturer: Scan" there, your warranty's with Scan, not the real manufacturer. Obviously if it didn't say that there and the manufacturer would not honour the warranty that would be different.
And it seems to vary whether it's a good thing or not. Some products end up with a longer warranty through Scan than direct from the manufacturer. Others definitely much shorter: the Seasonic X-650 power supply is 5 years elsewhere, but only 2 through Scan.
I agree that it does leave an uncomfortable feeling, as doing it seems to be fairly unique to Scan.