What Robin said is exactly my experience.
Liberty had nerve damage and arthritis of the spine. the dog does not know the poo is falling out. That's why you have very clean dogs leaving poo behind. It especially happened when Liberty went to get up. The vet explained the biomechanical reasons for that.
I learned to watch for the tail rising. She'd be sound asleep and the tail would start to go up. I'd grab a tissue and "make a catch". Done. Fortunately, her poo remained well formed (I said many a prayer of thanksgiving for that).
As long as she was enjoying life, it was fine with me.
Trust me, fecal incontinence is much easier to deal with than urinary.
Liberty only wore dog panties at night or when left home alone. Again, if poo is well formed, this isn't like a diaper. She'd go to stand up and the poo would fall out. It would essentially roll around in the panties. I'd take them off, roll the poo into the toilet and toss the panties in the wash (or even do a quick sink laundry).