Emissions Wokshop! warning help

Helo all new to this, so here goes, I have a Superb 1.9TD warning light message came on last night, Emissions Workshop! checked the user manual, not listed in there, so called into dealer this morning, advised if driving ok (it is) will book in for a check later. Came out the dealers, the sodding light and warning have gone off! any idea what the cause is? recently MOT'd and Full Service.