I haven't done a re-scan in a while so I'm a bit rusty, but...
When you're selected for a re-scan, our handset will either tell us to scan a number of random items, or to scan until we reach a random monetary value. So it could tell us 'look for 2 items' (which we pick at our discretion) or it might tell us 'scan items to reach a value of £18.50'.
What I think happened with you is that the handset didn't tell them to look for 2 particular items, it just asked them to scan 2 random ones.
If you had scanned those 2 items while shopping, you're good to go. If you hadn't scanned those 2 items our handset would tell us that you hadn't scanned them. It could be shoplifting or it could be an honest mistake - either way, it's flagged to us.
Fun fact: if shoplifters are caught in this way, we can block their MyWaitrose card, meaning when they try it again in a different branch, they can't.
Hope that makes sense