Apparently last night my dog pee’d on my sister without her knowing. My dog was sitting on her lap and just peed on her with no warning. My sister didn’t notice until she was getting ready to go to bed. My dog is house trained and 99% of the time when she has to go she tells someone. And even if she couldn’t get outside she’s pee on the floor, not on a person. Apparently this is the second time this has happened, both times on my sister without her knowing. The first time was about 4 months ago.
I wouldn't be terribly concerned if it's only happened twice in four months, just keep a close eye on her. Pat where she's been laying down to see if she may have leaked there too (it's easiest to notice on your lap). The fact that your sister did not notice also indicates it was probably not a full bladder because... she'd have noticed if it was. Trust me. ;)
There's a slight chance your dog has a bladder infection or something similar, which can make it hard for the dog to know or control when it needs to pee. My parent's dog pees by accident when she's played in the river and ingested too much water. If the dog starts peeing more and more regularly, definitely take her to the vet. A number of bladder issues are relatively easy to fix.