Hidden clothes is the fetish of going out in public while wearing a fetish garment concealed under normal clothes. This may range from a man wearing panties as underwear to wearing a polo neck catsuit under sufficiently concealing clothes.
As a variant, part of the fetish item can be visible but in such a way as not to reveal its identity. Thus close-fitting thighboots can be worn under trousers and appear to be normal boots. The lower part of a bodystocking looks like tights, and the lower part of a lycra catsuit like leggings or opaque tights.
There may be a thrill in having a secret, and anticipation of what the wearer will be able to do when alone.