OLD MICRA, occasional flat battery.

[moby]

Last year I sold an old (p reg) micra automatic to a neighbour, It had a new battery fitted and ran very well for its age. But now and again, say 4-5 months apart the battery is flat in the morning, and I mean flat.!! so flat the radio loses its code and there is nothing on the dash at all.

The battery tests out OK  nothing is left on, have checked the boot light, radio, which is ignition controlled anyway, sidelights, in fact all possible suspect drains, but nothing, it is now getting a bit embarrassing because I don't know what to try next, ? any ideas.??