My smooth Collie girl, Paris has this issue... and honestly we have never really been able to find a reason.

She was probably around 3yrs old when it first started (she'll be 5yrs old this sept)...But at this point in time she would leak urine when she was just walking around the house. If she was relaxed or sleeping she would do it worse.

Took her to the vet, did a UA, EVERYTHING showed up normal. Put her on antibiotics just in case, but they did no good. It would come and go. Took her back, this time we did a urine culture and again that came back perfectly normal. Bloodwork normal... everything was normal.

She is NOT spayed, which is even weirder, since spayed bitches tend to have this issue more often.

Good luck.. hopefully you will get some answers. Never got any real answers on Paris, other than possible weak bladder muscles, and I could have put her on DES or Prion to control it, buts not something that happens 24/7 so, so far I haven't done meds for it.