If you don’t see it does not mean anything except you can’t see it .
You can’t prove something is not there simply because you can’t see it .You would not be proving a negative , just an absence , bit like your intellect .
External level too high and exasperated by the single course of bricks with thick cement on top , no sign of dpc, or weep holes which are especially important with an inward opening door.?
I fitted a fan in third floor bathroom , used a length of string thru the hole to pull up outer grill and fixed with silicon thru the hole and cable ties to hold in position.
Pair of kitchen wall brackets with one reversed would work . Or use the plate you have and only fix at top and bottom holes, back off screw half a turn which will allow lateral movement .