all they can really do is isolate circuits until the problem doesn’t occur anymore.
You coudl do the same by unplugging fuses and seeing what happens. Could be a long process though.
They should be able to actually test for current flow with everything ‘off’ though.
have you got a small child you can shut in the boot or glove box to see if the light stays on? It’s likely to be something like that.
My brother had a bmw that the battery constantly went flat on. Turned out the alarm had some new fancy gubbins in it that put more load on the battery than expected. The bmw main dealer just fitted the biggest amp/hour battery that would physically go in the tray and it ‘solved’ the problem.