Another character works for the non-emergency police enquiry lines for a UK constabulary, present day. This is a job I used to hold myself so I am very familiar with pretty much all the little procedural details involved! However, I have forgotten one small detail which is crucial to an aspect of the plot.
How long does it take for the police to take action on a missing person, (ie to class the person offically as 'missing') - from the time of the disappearance being noted/reported? I'm aware that this varies with the age/situation of the person. I know that there are, more or less, guidelines for when the police say 'we'll treat it as a misper'. My 'misper' is a 'normal', psychologically, physically and financially secure adult who fits none of the police's listed 'risk factors' and is otherwise not really your typical candidate for disappearance. I am aware that in some cases, a perfectly safe adult might not be treated as a missing person until a few days have passed - after all if a person wishes to go away for a night or two out of contact of friends and family, s/he has every right to do so. Police resources are stretched and I'm fairly sure sometimes people are told to wait to see if s/he comes home at first. The missing person in my story will be eventually classed as such *only* because of the length of time s/he has been missing. But how long does that take?
Things googled and found:
I have googled 'missing persons' 'missing persons procedure', 'missing for five days', 'missing adult', and all sorts of other combinations within the UK advanced search. Though I can find plenty of information (for anyone who wants it, this is a very useful 133-page document) I have no actual timescales. The document linked only tell us about risk assessment, and procedure from the point that the police begin investigating. I am loath to call the National Missing Persons Helpline as they are a charity who don't want their time wasting. I don't want to call police non emergency enquiry lines, because I know what sort of response timewasters get! Also I think it looks extremely suss and personally if I got this call when working on the enquiry lines, I would at the very least report it as suspicious.
So. Any help would be very useful. Thank you again, wonderful community. :D
[EDIT] Thanks to my very useful three commenters, the answer more or less seems to be that the police consider 48 hours' disappearance sufficient waiting time. And I have learned my lesson: I will bother the police's PR department before coming and bothering you guys. THANK YOU!