Yes there is room in a cap .. but what consists a hypthetical threat in the eyes
of security creatures is beyond comprehension anyway.
I like to wear a very light scarf on planes which do not allow to control the
amount of airflow (such as some 763).
Every single time, I have to put the scarf on the belt. Always being told -
politely - that the scarf could conceal an item.
With that argument, we could all end up in underwear or worse.