How to get dog to stop peeing on sofa?

As the title says, my two year old lab mix will not stop peeing on the sofa. She was house trained by about 6 months old and then went through a brief phase of peeing on my older dog's stuff. She went months without an incident but now she pees on the sofa every couple of weeks. Just the sofa. I tried leaving pee pads in the living room and she ignores them. She almost always does this in the evening when I'm in the shower or otherwise not paying attention to her. Any suggestions would be very welcome. Thanks!