I read somewhere that chewing different flavoured gum whilst revising for a topic, and then chewing that flavour again in the test helps you to recall your revision. It's all about creating associations which work for you. For example, in spanish the word "cara" means face. I remember it because I associate the word "cara" with model Cara Delevingne who is famous for her face. Rather than associating words/topics with people, like I do in languages, to help you remember I guess your brain could also do it with smells or tastes. But I wouldn't risk smuggling gum or scented tissues into the exam. Instead, revise with flashcards or past exam questions which help you to properly store information in your long term memory without needing physical triggers, Good luck