Just using "/roll" to find something illegal really would ruin some peoples RP experience, as the LEO could end up rolling a higher number than the person in the vehicle/property which may result in the person owning said property to be arrested against the way they wanted to RP stuff out; in this case RPing as a normal citizen with nothing illegal. So I don't really think /Roll could work here, if anything it could be abused by LEOs.
Police Dogs are trained IRL to sniff this out, I'm not sure the chances of them not finding it in a place that has it, but I'd assume extremly low. This would be RPed out in "/me" by the owner of the property (Property being a vehicle, house, etc.) saying if they had any illegal substances or not. However, that doesn't mean /roll couldn't be used here.
This is something that normally would get RPed out anyways using /me and the other person would reply using /me giving a number or just saying if they were/are over or under the legal limit. "/Roll" really wouldn't work in this instance.
+ Last I checked SCP-RP had a /roll system, so this wouldn't be a thing that couldn't be implemented.
+ "/Roll" Could still have many other uses in Police-RP. For example, It could be used for RPing baseball games, and **NOT ENCOURAGING PEOPLE TO DO THIS, BUT** for slots, AKA gambling.
- Could result in some forced RP situation, for example in traffic stops is anything illegal found in the vehicle.
- This directly relates to con listed above, could result in more staff calls, because of forced RP. (Example listed above.)