Keyless Go fault finding

Hi all -
I need to do some long-distance fault finding on my daughter's 2006 K12 as it (and the manual) are in another part of the country.

A few weeks ago the red immobiliser light occasionally remained on when she tried starting the car. She changed the fob battery which may / may not have made a difference (difficult to know as it was intermittent).
In recent days she says if has got worse and happens most times she tries to start, but if she gets out of the car and locks / unlocks it then tries again it always starts. Locking / unlocking from inside the car does not help though.

So...what is the immobilizer detecting in the key, is it passive or does the fob battery have to be working?
Any thoughts about what could be causing an intermittent problem?
There is only 1 key btw so cant just try another.