High level brake light coming on when door slammed

[oh752]

Hi All

Could anyone help please with the following? Car's a 2006 XC70

When car is knocked (i.e you knock on a window) the brake lights come on - sometimes all, sometimes just the high level one. They also come on at random when driving

Any ideas where to check first please?

Does anyone know where the relay box is for the brake lights too? It ain't in the back like my old 2002 V70

Thanks a lot