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 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.
Any worker would tell themselves it could only be the (ArcelorMittal) management, but we want to have certitude on that, a spokesman for FGTB told Reuters on Friday.
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? he said.
The union said it had received a disk containing footage of workers at the ArcelorMittal site in Liege in 2008 and 2009.
Last year, six ArcelorMittal executives were held hostage by workers for 48 hours after protests in Liege intensified.
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.
(Reporting by Ben Deighton; Editing by David Cowell)