Hi there,
The first thing I thought about when I discovered I had HPV was my husband cheating. I have been with him for going on 8 years and he was my only partner. We both to my knowledge have been only with each other since we have been together. He had multiple partners growing up though before me...my only thought is that I got it from him. Now did he get it before or did he get it recently (not sure) but the doctor says that he could have had it this whole time and it was lying dormant in his system and mine until now. I have had several paps prior and checked all clean until now. My instincts say it is something that is just coming out after all these years. My doctor says it is a good possibility too. I decided to go with it...trust my partner...and take it day by day. Ultimately it is a decision you have to trust yourself in dealing with. Good luck to you!
It could be that the test was unable to diagnose HPV because it was laying dormant in her system. She could have caught the virus from a previous partner 26 years ago and due to good health or luck the virus lay undetectable until now. Though this is unusual, it happens. It's definately more unsual in such a long amount of time.
Other than that, you could of got it from a previouse partner 26 years ago, gave it to your wife unknowingly and the virus only just became active. Or she only just got it from you because it was laying dormant in your system all this time and only became active.
The last option really, is that she wasn't faithful. Consider all of your options and good luck.