Why is your husband so secretive about money issues in your relationship?

A spouse hiding money is lacking in integrity towards their marriage. It's compounded if you're seeing other red flags---changes in schedule, unusual phone calls or behavior on your husband's part--that are suggestive of an affair. You can't have a relationship that survives without mutual honesty and trust. Lying and secrecy add (as you've discovered) to resentment which will fester and grow. For the time being, keep your eyes open and be prepared--discovery tends to trickle in. Just gather up what you have, and stash it away. When you finally reach the point where you are convinced he is having an affair, then it will be time to take some action. Keep quiet as well; if your husband figures out what you are doing, he will simply go to further lengths to bury his activities.