Is your audience B2B or B2C? If B2B, they have sophisticated spam filters testing your emails, "clicking" into your answers giving you false responses. We found this out the hard way.

Thanks for the response. It's B2B, so that might help explain it but is rather worrying. How did you find out which ones were false and which ones were real?

There is no way to know, unfortunately. You will get false responses. The only way we found out is through closed loop process - we were calling customers to follow up on their "negative" feedback and they said they never even opened the email. Our security team advised us on how anti-spam tools worked. We remove inline questions from all our survey invitations after that. Now we just have an inflated "partial response" rate on our dashboards because anti-spam tools are clicking into the survey link and Qualtrics is registering that as a partial response, but at least we are not getting false responses in our data.

Hi InessaG
I raised this with Qualtrics Support and they came back with the following:
In this case there is an option, the Survey options > Security > 
Fraud DetectionSecurity Scan Monitor This will Prevent security scanners from accidentally starting a new session on your survey. 
Have you tried this? And if so, did it give you confidence in the results?
Thanks
Lisa

I believe I always have that option on.
How would they know that this is not a real person? A click into the survey is a click into the survey, and it gets recorded. The option they mention is useful only for anonymous surveys, I believe. It makes sure that only one response is submitted per IP address. If the links are unique per respondents, they'd have no way of identifying this unfortunately.