Our church's baptismal, IIRC, was filled with a hose coming from a nearby (but hidden) spigot. I know the church's hidden areas better than pretty much anyone but the pastors and possibly the elders, but that area's far out of the way. On the left side of our sanctuary, behind the walls, is a hallway accessable through two doors that nobody would ever bother looking inside of. At the end of that hallway is the front corner of our sanctuary (it's a semi-circle, not rectangular) and behind the screen where the lyrics to the Jesus songs are projected during worship is where the baptismal is located, so it's easy to hide stuff by.