The source for this is Unite the Union:
Unite understands that finger recognition technology is being used, with workers asked to touch a happy or sad emoji as they clock in to indicate whether they feel they are treated with respect.
According to Unite, workers selecting the sad face are asked if they are sure. If they are, pressing the sad emoji again results in the worker being called in by management to discuss why they pressed it.
The Guardian also has an article about this. It also contains a statement by Sports Direct which doesn't deny the existence of this system, but does say that Unite misrepresents their intent.
The Daily Mail article may be a bit unclear. Workers seem to have known that they could be identified when pressing the button (see the Unite press release).
According to Unite, the reason for this system was not to secretly identify unhappy workers, but to show that their workers actually were happy (they have previously been accused of abusing their workers), and to get those workers that non-anonymously declared that they were unhappy in touch with their manager.
Even so, how would they link those to employees?
They already had the fingerprint of employees because they use it to sign them in and out.