False plates??

Hi peeps, cut a long story short ran these reg plates through the goverment reg check and they came back as not recognised?? Is there ever a legitimate reason for this?? They weren't private plates.

• There was a proper shifty looking pair of women who pulled up outside the shops we were at, in a no parking zone, left their engines running, put their hazards on and opened the doors. Left their 5 year old in the back whilst they made a beeline for the shops, like they were literally going in to shoplift and make a quick getaway, so before we left the car park out of curiosity we ran the plates to see if they were legit, and yeah shocker, no details held/not recognised?!

Were they running false plates??