I guess I'm a little confused... In your first post, I took it that she watches it all day every day while you're at work. But then it seems you've only found little bits and pieces of evidence once in a blue moon. If it really is only once in a great while that she looks at porn, maybe she's not lying when she says she doesn't have a strong sex drive. And when the urge does come up, maybe she just wants to take care of it and be done. No pressure type of thing.

I know you said you don't want to talk to her about it, but I really feel that would be beneficial for your relationship; especially if there are these long dry spells that are leaving you unfulfilled. Maybe she's surprised you haven't brought it up. At the very least, you should probably have a calm, neutral conversation about your sexual relationship, if she's happy, how you're feeling, etc. I think the key to any relationship, and to all aspects of a relationship, is communication. Maybe romance her a little, get her in the mood, and afterward start a conversation about her likes and dislikes, just to start getting her more comfortable talking about these things. Ask her if you two could watch together sometime.

Good luck, I hope everything works out for you. Sending good thoughts your way.