Limp Home (?)

Hi

Test drove a Discovery Sport 2015 (22K) on Saturday. Pottering along and asked the guy if I could 'test the engine'

Foot down, very little acceleration, no warning lights. Sales person said it was due to low fuel (~7 miles).

Topped up and it did seem to resolve the issue but I can't find anything online to confirm this is the case.

Thoughts? .. or walk away?

Call me suspicious, but it could have an unrelated and potentially expensive restricted power mode fault. On my Volvo at least these can sometimes be reset by turning off the ignition, so a cunning salesman could test drive it with minimal fuel and if it occurs, blame it on the level and re-set the system by stopping for fuel.
OTOH you don't mention which engine, but the Ingenium engines in particular could suffer fuel pump damage if run dry, so are designed to give a low fuel warning with a good 50+ miles remaining, and will probably shut down before running dry. A restricted/economy mode at very low fuel levels would make sense, though I cannot find any documentation for this either.
Was this a JLR dealer? If so, ask to see a new diagnostic report and test drive again with a decent fuel level. If they don't agree, or this is at a suspiciously low price in a used car dealer forecourt, then there are plenty more out there.

Thanks for the advice .. it was a JLR dealer but gonna leave it.