I am rather surprised that your curiosity did not drive you to undoing the lower mounting bolt to determine how the footrest was attached. Mine did.
You would have found that the lower hole houses an M8 x 20mm bolt which holds the footrest in place.
There is a grooved stud (my vernier measured it as 6.88 mm outside diameter) welded to the footrest baseplate which passes into the upper hole of the footrest. The footrest has moulded into the bottom of this hole some plastic lugs which I presume are intended to grip on the projecting stud because my car does not have a nut on this stud. (As an experiment I tried an M8 nut on the stud but it was far too loose and an M6 nut was too small. I can never recall seeing an M7 nut though I am sure they exist).
So the answer to your question is that you do not have a nut missing.
My footrest is quite secure and does not feel loose even with the lower bolt slackened a little.
Footrest Baseplate
Gripper lugs moulded into the base of the upper hole in the footrest.