I respectfully disagree with the last post.
First of all, trust your gut instinct. In matters of the heart/infidelity, gut instincts are usually on, or close to the mark.
Secondly, a healthy marriage has NO secrets. No hidden phone bills, credit cards, etc. If there are no lies, there is no need for secrecy.
If you have some of the old cell bills, you can access his account on-line. With many accounts you can see all incoming and outgoing phone numbers, then cross reference them with Anyone Anywhere, cross reference system on line.
If he cannot be accountable for his time, and honest about his whereabout, that is another red flag. Does he appear to be unattached to you in the past few weeks/months? Do you know where he is spending his money? Check his credit card bill for individual purchases. Gifts you didn't get? Meals for two your weren't present for. Motels? How far to work and back for him? Check his mileage and see if it stacks up. Check pockets in his coats, suits, glovebox in the car, under the seat, athletic bag. Look for more clues, some that might give you a better idea what he might be up to.
I hope you find nothing! I really do. But if he is up to something, it is best to have really good, concrete evidence that he is, before any confrontation.
Purchase a copy of Surviving an Affair. It has good information on how to make a marriage "affair proof", or how to deal with a spouse who is in an affair. Either way, it will either help your marriage, or help you SAVE your marriage.
Hopefully this post will give the thread a bump and you'll get some other responses.
Good luck
SD