In the 5e Player's Handbook, rules are given on hidden weapons with sleight of hand. However, it doesn't say if there is any benefit to the hidden weapons or what it should be like.
Can a hidden weapon cause Suprise if combat is initiated with one being drawn?
For example, assassins might be disguised at a ball. They aren't hidden but it isn't known that they have hidden blades. The nearby player characters' and guards' passive Investigation is only high enough to notice one of the four assassins hidden weapons.
Are there rules from previous editions that might help?