No, the plug is supposed to stay. The drain was clogged with dirt or something. Isn't necessarily common, but it's much more common on trucks that spend a ton of their life on gravel roads and such.
High mount brake light seal was also problematic. My '04 Lariat leaked there and filled the wiring tray under the door sill trim. When you took off you could hear the water sloshing to the back and then when coming to a stop, it would slosh to the front.