Engine alert?
 

2004 SAAB 9-5 2.0t
Estate. Off and on over the last year the little "engine warning" icon has appeared on the dash but usually if I stopped the engine and then re started it, it went off. For the last three days now it has remained illuminated. There doesn't seem to be anything wrong with the engine, just come back from three days touring Cornwall with no problems, just the "warning".
Doe anybody know what this is trying to tell me please?