The gap between when it was built and when it was first used here
Where to look: Year of manufacture (DVLA) vs first used date (DVSA)
DVLA holds a year of manufacture and a month of first registration. DVSA separately holds a manufacture date, a registration date and a first-used date. On a car sold new in Britain these sit within months of each other.
A car built in 2015 whose first UK use is recorded as 2019 spent four years somewhere. The overwhelmingly common explanation is that it was registered, driven and sold in another country first, then brought here.
The size of the gap is informative in itself. A few months is ordinary. Three or more years, on a mainstream car, is not.
When it is not an import
Unsold dealer stock and pre-registration games can put a year between build and first use. So can a car that was stored, an ex-demonstrator held back, or a low-volume model that sat unsold. A gap is a question, not a verdict.