is 秘 even used for normal secrets?

ok so my flashcard says 秘 means secret but i tried to use it to tell my friend i 'secretly' went to the store and they had no idea what i was saying. is 秘 not a word you can just say by itself? like do you have to pair it with other kanji? i thought i could say 'i did 秘' but maybe that's completely wrong. is this dumb or am i missing some major grammar rule idk...