I don't think he/she was hiding in the drain outlet, I think its more likely that the drain acted like a pit-fall trap and he probably decided to stay a while. I'm glad you saved him from the hot shower though!
To stop a recurrence, of this I would suggest fitting some wire mesh or a plastic cover over the drain.
I remember finding toads 6-8 inches under the soil a couple of times and also under a heavy flag stone, so he may have been around all the time, found them in stopcock holes too at times.