It's frustrating, isn't it? I have had blood in my urine since I was dx'd with SLE in 1994. Sometimes it's just a trace, other times it's >2.
I, too have seen every doc imaginable, but none can figure it out. So, it's still a mystery. My rheumy doesn't think it has anything to do with the lupus, since my kidney function is normal.
But my mother-in-law, who also has lupus, has had blood in her urine for decades, without knowing the cause.
I know this doesn't help, but wanted to let you know you're not alone.