Was there ever a rumour of similar disappearances in the area? Extremely damaging to (what looks like) a purpose-built tourist resort, so the town itself had a lot to lose, from news of a child-kidnap gang in the vicinity.
The preliminary police handling ought to have captured a 'snapshot' of who was staying (and where) in the resort and all overlooking properties, some of which might not have been for rent to tourists.
If I recall rightly, by the time of the smuggling ring rumour, data of who was where was, variously, mislaid, destroyed (having proven to be investigative dead-ends), vague on residency dates, or *not gathered in the first place*.
Unavoidably, all available manpower must go into the search, not into door-to-door data gathering, or ensuring renters and locals do not slip away, in the night. I defy anyone to do the job effectively, with the resources available to what is/was a semi-rural area.
(That's not intended to read as an excuse, just my guess as to why pertinent facts were not captured before their importance could be realised).