I don’t have an answer for why this is happening, but I can tell you I’ve experienced the same thing over the years.
Started when I was in my early 40’s, I’m 65 now.
Same as you, I went through a battery of tests which did not show any reason for the blood.
Then over the years, my urine analysis would sometimes show blood. The doctor would retest in a month or two and the blood would be gone.
Just this February, while away on vacation, I had visible blood in my urine, very light pink. Since I was out of the country I did not feel comfortable going to a hospital, so I increased my water intake by quite a bit.
I saw my doctor when I got home and an initial urine culture showed I had a bladder infection, but further testing refuted that.
So I am back to square one. I have no illnesses to speak of, except osteoporosis, so no idea why this is happening.
I feel fine otherwise.
My gut tells me that if anything was wrong it would have shown up in one of the many tests I’ve had over the years.
You can’t fight an invisible foe, as long as you feel good and your tests are normal, don’t worry on it too much.
Just my opinion.
Take care,