User clicks link to unique URL, but not found in logs, why?

apache log analysis question regarding tracking and user requests

[webplacing]

Hi All,
How could this occur:

User clicks on link:
www.domain.com/page.html?source=campaign-name

But this request is not found in the Apache logs anywhere.

Thanks in advance.

Mark

[webplacing]

bit more info:
I'm looking for the following in the logs:
"GET /page.html?source=campaign-name"

Is there anyway someone could click on a link to:
www.domain.com/page.html?source=campaign-name
Without me being able to find them in the way described above.