Walking within 5 meters as you walk past it won't do anything. If you pick it up then you are starting to get a concerning dose and need to get it at least 5m away from you and now that you know where it is you need to remain 5m away.

With how radioactive it is if it's along the road they will easily find it with radiation meters. If someone has stolen it then it will be hard to find, but not impossible.