2004 Grand Cherokee losing power steering fluid but no wet spots?

This has happened only recently, but I want to say there is a leak. Unfortunately, I can't find any wet spots in the system or under the vehicle. I put in some more fluid with stop leak but that only lasts a week or so. Does anyone have an idea of where I can check or a better idea of what to look for? Odds are there is an old O-ring somewhere, but without any evidence I can't say.