There are no definite rules about the use of sacred symbols. The rules are also quite silent to define what a Sacred Symbol looks like, or how big it is, or how it is used. The closest you can find is in the Team section, which is totally silent about the shape or size of a Sacred Symbol. A sacred wooden symbol does not have a significant weight, but the others weigh 1 pound, which implies that they are quite large.
Depending on the deity worshiped, hidden or deceptive actions can be encouraged, or they can be against the spirit of the deity. There are too many possibilities to definitely indicate one way or another, and in the end, it would be from your GM anyway.