Agreed with Paul, either pushed out by hot air or pulled out by cool air passing over and will raising the bonnet do similar?

I am tempted to add some air sensors for what it is worth, closed and then vented. Measure air under bonnet constant and air at back of raised bonnet and air at vent. If air at vent or back of bonnet is higher than under bonnet the surely heat energy is escaping somehow for the benefit of heat loss and if under bonnet temps do not alter then all is a waste of time?

Measure water and oil temps throughout

Does that cover all option?