Welcome to the forum, lets hope you do not have any real reason to be here, but you just ran across a major red flag. You have a few options on how to proceed, I'll attempt to give you some straight talk on them.
First and foremost you need to think about the last few months of your lives together. Has anything else been out of the ordinary? Has his behavior changed in any significant way? Has your sex life/frequency changed at all? (more sex, less sex?) Is his cell phone password protected? Do you have access to his e-mail, facebook, bank accounts? Has he been late or early coming home from work? Has he mentioned any new friends?
Option one: Verification and evidence gathering.
You ran across a major red flag. Your husband, who does not need to use condoms to have sex with you, bought a condom and had it on him. The only reason he needs a condom is to prevent himself from catching a sexually transmitted infection/disease. If you have a clean sexual health record, then obviously it was not to protect himself from you. There are a few steps you need to take. Put the condom in a safe place somewhere he can't find it. Go out to your local office supply store and purchase a few voice activated recorders (VARs) they can be found for under $40. Use velcro to secure one of the VARs underneath the drivers seat of his car, review the recordings every few days. When you take one VAR out of his car for review, replace it with another one. That way you have time to review the VAR in peace. Place another VAR somewhere in the house where he is likely to spend a good bit of time. The point is to catch him talking on the phone about inappropriate stuff and have evidence to back you up when you confront him. Secondly, if his cellphone is not password protected, and you can get uninterupted access to it for ten minutes or so, install software called Mobile Spy, if you use a search engine, you will be able to find the web site without issues. There are instructions there on how to install the software for any phone. Install one of these monitoring programs on the computer (Monitoring Software Review 2012 | Best Computer Monitoring Software | Monitor Activities on Your Computer - TopTenREVIEWS
) ive used spector pro in the past with some success. This will allow you to monitor what he is doing on the computer while you are not around. Monitor him for a few weeks, if you run across proof, keep it, confront him.
Option two: Confront him and bluff.
This is a more risky approach, and if it fails could drive whatever he was doing deep underground. Basically when he comes home, just walk up to him with the condom in your hand and say that you know whats going on, and you want him to tell you the truth. Carefully watch his response, this may also be a good time to have a VAR on you, so you can record his explanation about the condom and review it later, and have it for comparison should his story change. He may crack and confess. He may try and talk his way out, but still give you enough signals to cause doubt/answer your suspicion.
Regardless of what you choose to do, try and look over the financial records for the last few months. Try and see if there are any purchases you did not know about. Also go on-line and check the phone records for his cellphone. Look for numbers you dont know that he may be texting or talking to.
I hope that gives you enough of a framework to approach this situation. I wish you the best, and I hope I'm totally wrong about the whole thing.