it is seen by the customer, hence best placed on the Thank You page after a sale is made.
Sure, but is it seen by every customer not just those who have clicked on a Google Ad?
Google in AdWords Support under this heading "Why do I have to choose my site's language to track conversions?"
Say "When you place the conversion tracking code snippet on your conversion confirmation page and a conversion occurs via an AdWords ad, a small but visible image containing the text Google Site Stats will appear wherever you've placed the code. <snip>".
This could be interpreted as meaning not every customer sees it.