They have computer programmes that monitor Tax returns it picks up any changes that are significant to your return,if you have certain jobs that pay you on a regular basis including domestics and they stop or get paid to someone else it picks it up. Transfers debits direct debits transfers to other accounts they are all detected through the banks and the inland revenue have access to those records,if an account you have is lost or no longer showing in your records they will know. Altherythm computer software I was told by a customer the other day it records transactions and detects and highlights changes.