Immobiliser playing silly b's.....

My old '98 110 300TDI has no central locking but does have the old style 2-button fob where one button I believe should (or did) have a lock symbol on it? Anyway, sometimes I get in and put the key in and it starts fine, but other times the red LED comes on on the dash and I occasionally (but only occasionally) get the two short beeps on the horn. The stupid thing (apart from it seems completely random whether it'll do this or not) is that it doesn't seem to matter which button I press, either will cancel the LED and let me start it. Can anyone shed any light on this madness please?

Hmm just thought to check inside the fob and found the battery loose due to a broken solder joint on the little battery holder "cage". That could have been causing an intermittent battery connection which would or could have caused the weirdness, I guess I'll know in the next few days but it won't have helped though!