Firstly, SW cannot check out your bank balances within getting specific written permission from you.
Secondly, they have no way of checking out whether you have accounts or not in particular banks unless you give them writtne permission to do so.
Thirdly, if they find out in the future that you hid something from them, you will be brought to court and overpayments assessed if the fraud is large enough.
Fourthly, there is no reason to not delare bank accounts if your savings are less than 20,000, as nothing is assessed on this amount.
Finally, you will only lose about a Euro off youe weekly payment for every 1000 euro savings/assets value over this amount, so the risk is not worth it.