Outside of looking in /var/db/receipts there are generic printers in /library/printers for many different printer manufacturers. But since they are automatically installed as part of the original OS install--and this is only an educated guess--I don't think Software Update would be looking at those. (Although, I just reinstalled my HP software and I notice several new files at that location.) If it did, you'd be getting notices for updates for many different printer brands, that is, assuming other brands put out updates through Apple.
I assume you removed Hewlett-Packard from /Applications and from /Library/Applicaton Support.
Basically a long winded answer to say I don't know.