Adult Housetrained Dog Peeing in the House

We have a 4 year old Labrador retriever who is house trained. She is very good about going over to the back door and waiting for someone to let her out when she needs to go. Usually she sleeps on our bed through the night, but if she has to go she whines and we let her out. Recently, however, we have been waking up and discovering she peed in the hallway. Always in the same spot.

We have never seen her do it, so it's hard to know if she's anxious for some reason or in desperate need to go that she just can't hold it. She recently got blood work done and has been checked for a UTI. There doesn't seem to be a medical problem.

Has anyone else seen this kind of situation? I am trying to get an idea of the kind of behaviors to look out for to make sure she's OK. We're perplexed about this seemingly random new behavior.