Question about Invitation ID and tables

My understanding from the web side of Ultimate Survey is that the Invitation Manager generates a unique number for each invitation sent via the Invitation Manager.

Where is that stored?
Can the Invitation ID Number Only be linked to survey responses?

We had a bit of a mishap where someone inadvertantly sent out an invitation link via regular email, and I would like to check to ensure the link has not been used by anyone to submit a second survey. This was meant to be a reminder, but someone sent it outside the system to everyone. We get so busy this time of year around here, some folks do not even remember the original invitation sent a week ago.