This is so frustrating!
I'm having this issue with my daughter, who will be 4 in a month and a half. She has a sister who's two and a half and we just had another daughter 6 weeks ago. I know adding a baby to the house is a big change, and the baby gets lots of attention, but my husband and I are careful to still give the older girls lots of attention. She pees in toys, on toys, on books, on the bath mat draped over the tub, and on the carpet in her bedroom. And she has this proud glow about her after she does it, like she just did something exciting, or that she's proud of. So I can usually tell that that is what she just did. But I have no idea how to respond. The bathroom is directly across from her room. So proximity is not an issue. When I ask her why she chose to pee there, she just kinda glazes over. I don't think she knows her reasons, or at least cannot articulate them. This started before her baby sister arrived, but it has definitely gotten worse since.