I assume bitcoin full node: the full node stores all pub keys, that were used to sign a trx (that found it's way in a block), in the data structure in the home dir. Why? Because all vlid trx are part of a block, and all blocks since the genesis block form the block chain. So there is even more than pub keys stored on your harddrive of a full node. – pebwindkraft – 2017-05-16T10:13:06.613

Chapter 5 of Andreas book "mastering bitcoin" explains tx handling, and developper info can also be found here; https://bitcoin.org/en/developer-guide#transactions