Hi Springs
I'm a British Police officer and this is the general procedure when we get a Missing Persons report. (Misper). This is standardised and is used across the country.
1. We gain as much information from the informant as possible, why they think they are missing (as opposed to gone down the shops, on a booty call etc). Is it out of character etc.
2. We visit the location they are missing from, check it top to bottom to see if they are hiding (often the case with young children, they sneak off playing hide and seek and fall asleep in a cupboard), check for notes etc.
3. We assess any risk factors if they are missing, age, mental/physical health, drink/drug dependency, ability to engage with people, have they gone missing before and come home unharmed etc etc.
4. we grade them low medium or high.
Low requires basic checks of hospitals etc and not much more. The sad reality of these is by and large they are adults who are playing away from home and the informant is a spouse.
medium requires active investiation, we will go to known locations where that person is known to frequent. Checking borders, things like that. Much more active and there will be an officer detailed to keep going until theyre found, or we simply exhaust every avenue. then they will move to low risk.
High is someone who has threatened suicide, a young child, someone with severe physical mental disability. These are literally all hands to the pumps. If, for example a child goes missing in a town centre then we will call in extra resoure, get as many people as we can in on it.
The system we use is, depending on the force either called Impact or Compact, but they are both exactly the same.