A private house has a small, stable set of people who can open it, and the whole discipline is about keeping that set small and stable. A guesthouse cannot work that way. Over a season it will issue keys to hundreds of people it has never met, alongside cleaning staff, kitchen staff, maintenance contractors, relief managers and owners who may live somewhere else entirely. Every one of those is legitimate, and the total churn is enormous.
The mistake we see most often is treating that as a housekeeping problem — a matter of chasing returns and being firm with guests. It is not. It is a design problem, and the properties that never have trouble are the ones where the keying was laid out so that a missing guest key affects one room, costs very little to reset, and touches nothing else on the site. Where that separation was never designed in, a single unreturned key can compromise a whole building, and the remedy stops being a quiet ten-minute job and becomes a full lock change services exercise across the site.
The separation that matters most is not between one guest and another. It is between the guest layer and everything else — the owner's own quarters, the office, the store, the cellar, the plant. Those should never sit on anything a visitor holds, and building that structure properly is what master key systems are actually for in a hospitality setting.