I suggested the keylogger - but only if it is installed on a computer that belongs to the OP. Then it isn't illegal. It would be if it were on HIS computer or his phone, however.
Only speaking from experience, my ex had several hidden Gmail accounts for his fake FB profiles. I also came across these quite by chance. He also deleted his browsing history because he was hiding a fair bit, some of it was criminal stuff.
It is possible that the OP caught a glimpse of the new Yahoo layout, it has changed a couple of times recently I believe?
I wouldn't go snooping unless I had suspicions. If someone did not give me reason to suspect something odd, I would not be looking at all. In my case, I had good reason. My ex lied to me about something serious, acted very suspiciously and I found out he was lying by "snooping". He even continued to lie when presented with evidence. And he had deleted history showing his searches showing cheating as well as criminal activity - but had no idea I could still see those searches. Continued to deny it.
Again, I would not have looked unless there had been good reason. And it helped me make my final decision to ditch a cheater and a liar.
In OP's case, she seems certain she saw something and is being lied to. Most liars, if you ask them, are hardly going to just suddenly decide to be honest, right? They have too much to lose.