Only just seen this. My dog went through something similar a couple of years ago. Out of the blue I noticed he was "asking" to go out every few mins. I followed him out and urine was just dripping out.
Rang the vet and got an appointment. Urine sample came back fine etcetera. Prostate was fine. It turned out he had crystals in his bladder. I forgot the ones they where but they're the ones that unfortunately don't dissolve with treatment.
He had to be sedated and have his bladder manually emptied with a large catheter to allow the crystals to pass. Thankfully it worked as if it didn't he'd have had to be opened up.
Samson now is on a special diet to help stop the crystals forming. He still gets a burger treat a few times a month. But unfortunately that's about it as I really don't want him to get the crystals back.