Saw an interesting looking article on the above topic here.
I don't think many of these are applicable to SL; for example "Income tax returns that reveal unexplained or previously unknown travel-related deductions" does not apply because there are no travel related deductions (and hardly any other deductions) that are allowed.
Credit cards are the main thing to watch for here. I was told by a bank employee that they have had several instances of trouble when verifying overseas usage on cards-the bank calls the contact number (which, unless the person has roaming, is a home number) to verify a transaction and it turns out that the spouse of the cardholder is not aware of the trip.
Credit card statements also leave a trail, as do phone bills and info on both is much sought after by jealous lovers trying to nail down an infidelity.
On maintaining a separate bank account- this I find surprising, I would expect most people to continue with their own individual bank accounts even after marriage, in addition to having a joint account. I know someone who closed everything and put all her money in a joint account and lost a packet when her husband left. Sharing is the key in marriage, but don't the individuals continue with at least a part of their former lives and their bank accounts? What is the usual practice with managing money post-marriage for couples here?