Well, I think there are two things to consider here.
One is that there's no telling how recently Leah's (hypothetical) continued existence came to light. The comments the SIO made about her perhaps going voluntarily and still being alive are quite recent iirc. Likewise, her parents' complaints about not being allowed to sort out her bank account, even though we know there's a mechanism for them to have done so if granted the right, are also comparatively recent. So possibly there's been a shift in the investigation's status only recently. I certainly think there's been a recent shift in tone. A lot of people here have commented that she would be unable to earn and support herself if she voluntarily disappeared but maybe she's only just wanted/needed to, which might chime with the widely held belief that she might have been pregnant when she disappeared. If she registered a name change with HMRC before disappearing (probably on the advice of someone else) she'd be able to resume work under her new name (i.e. without alerting an employer) but in all likelihood TVP would have a flag on her tax and NIC account, which may only recently have been triggered.
The second thing imo is to consider where the pressure's coming from to continue investigating, and the answer to that is clearly her family. Friends, former colleagues and MK generally do care and are interested, but it's unlikely anyone is calling TVP every week/month asking what they're doing to find Leah lately. Her family are, though, and are entitled to answers. But what if - again hypothetically - Leah's family is who she was running from? If TVP are at a duty not to disclose her continued existence, and are simultaneously at a duty to respond to requests for action from her family, that's a very difficult line to tread. If they refuse to take any action at all on an investigation that everyone believes is still open, then that in effect ends up being a disclosure in itself and a breach of their duty to Leah.
I don't think we can know whether the investigation is formally open or closed, and I imagine, though I don't know for sure, that in this situation very few people would know. If its status is DND, then a FOIA request will be fruitless.
I wouldn't personally say that recent efforts have been that resource-intensive. It costs very little for the SIO to talk to the local paper as part of an anniversary/birthday appeal, and by using social media they can recycle the appeal poster periodically without even paying for photocopying. Afaik, nothing costly like door-to-door enquiries, dredging bodies of water or scouring hours of cctv footage has happened for quite some time. All that's happened lately is that they issued a statement explaining why the sighting of the driver in the leather coat wasn't treated as significant, and confirmed to the MK Courier that they could print the story about the red thing in the Eaton Bray barn (in effect confirming that that wasn't significant either). We ourselves have wondered how they can be so sure in both cases, and perhaps this is the answer.
This is only one hypothesis and it's not perfect, but for me it's way more plausible than that some guy at work who may or may not have had sex with her would engage in an elaborate plot, involving multiple people any of whom might screw up forensically or when questioned, to abduct and murder her as part of a so-called honour killing. If Leah was as throwaway to X as many people believe, I can't see him taking such enormous risks for her. On the other hand, as I said upthread, I know personally of someone escaping a domestic situation whose disappearance has been handled in exactly this way. The only difference here is the higher profile of Leah's disappearance, which presents the police with some additional, but not insurmountable, practical problems.
JMO as always.
