Belgium's heavy industry has been in decline for years and Liege has been the scene of acrimonious protests by workers after ArcelorMittal, the world's biggest steelmaker, said it planned to close operations there.

The discovery of a camera pen
The socialist FGTB union, one of Belgium's biggest, filed a complaint this week with the Liege court following the discovery of the camera pen, and is looking at filing another based on the latest footage, which it also received this week.

A spokesman for FGTB told Reuters that "Any worker would tell themselves it could only be ArcelorMittal management, but we want to have certitude on that. This is about filming people with relatively sophisticated equipment so people don't know they are being filmed, so for what reason, and who has ordered it?"

Footage of workers
The union said that it had received a disk containing footage of workers at the ArcelorMittal site in Liege in 2008 and 2009.

Read more: A blow for the Belgian steel industry

In 2011, six ArcelorMittal executives were held hostage by workers for 48 hours after protests in Liege intensified.

Read more: Poor outlook for the Belgian metals industry

It said last year it would permanently end liquid phase steel production at its site in Liege, Belgium, given over capacity and a slow recovery in the European market.

ArcelorMittal said it had not requested the surveillance reported by the union and had ordered an internal inquiry into its internal security operations.