Hi Sven.
When I first had my S3 I found an ignition key screwed to the back of the centre panel in the same position as yours, held by a small Terry clip which was at that time unoccupied. Not knowing the function of the clip I also wondered if a previous owner had hidden it there as it seemed a crude way to hold a key. (The clip now holds the previously hidden heated rear window fuse, thanks to a recent S2 thread revealing its purpose).
As mentioned in a post above the reported place for a spare key to be found on an S3, or at least a hole where it had originally been fitted, is under the bonnet.
Sometimes facing one way
and sometimes the other
But according to Service Bulletin A.39 it seems that Jaguar started providing 2 spare ignition keys for the S2 around spring 1970, one behind the instrument panel and the other under the bonnet although no Car Numbers are identified as the start point.
This thread prompted me to remove my hidden key to check it worked in the Britax lock. Yes it fits and is a Britax key but no it doesn't turn in the lock.
