While there is evidence still to view/check out/ have examined by the relevent expert, they will indeed have kept going at it for those 3 or 4 years, albeit it might get allocated to one D only to plug through.
It won't get filed as undetected and parked in the cold case queue for future review until all the available evidence has been exhausted and there are no qvenues of investigation remaining, at least the the two forces in which I worked sarf of the border it wouldnt.
Just before I left in 2017 I did some work on one from 2001 that was still active. One skipper running it only, and borrowing in staff with expertise as needed rather than having her own staff assigned, but it was still ticking and it won't stop while there are still avenue of investigation.
Meanwhile they're wasting time on officers beards and having to pay compo to four officere that dug their heels in and started tribunal proceedings (one of the four has just retired and has joined the IODPA and has been telling us about it.) Nice to know that my home farce has their priorities as jarked as the plod down here.