The spa keeps a paper diary. A resident guest books two treatments on Wednesday and asks for them on the room, which the therapist notes in the margin.
On Thursday the guest checks out at seven in the morning, before the spa opens. The margin note is still in the diary. The property has delivered £180 of service and billed none of it, and will not discover that until somebody reconciles the diary against the folios — which, realistically, nobody does.
The same shape recurs across every department that operates outside the front desk's systems. Laundry collected on Tuesday and charged on a slip. A tour booked through the concierge. A late minibar restock that was never posted. Each is small; together they are a percentage of revenue that simply never arrives.
What removes it is not more diligence. It is that these departments post to the same folio the restaurant and the front desk post to, through the same billing anchor and the same tax engine — so a treatment marked delivered is a charge on the guest's bill, and checkout can see it.
A walk-in settles at the counter; an in-house guest's outlet bill rides onto the room. Either way the money is recorded at the moment the service happens rather than at the moment somebody remembers.
Why this matters to you: ancillary revenue is where an independent property's margin actually lives — rooms cover the costs and everything else is the profit. Losing a slice of it to paper is losing the part of the business you have the most control over.