Well I wouldn't think too much because a client is supposed at least to take a shower when arriving and/or when leaving.
So before thinking about the SP shower there is lot of opportunities for the shower to get wet lol
Also I guess this situation would be different between agencies/Indy, incall/outcall, etc.
You also don't know when the provider has arrived at the incall.
If it's allowed I personally like to show up an hour before the booking. Most providers arrive 30 mins before.
Maybe she's been hanging out at the incall the entire day. Some people just hang out at their incall for a few hours.
And you are right - if it's an agency incall, if it's not the first call of the day, it makes no sense for the shower to be dry because not only the providers shower but also the clients.