As a first step, I recommend that you confirm the rumours by having a fact-finding team in place. Ask them to look through various forms of evidence to confirm if the rumours you have heard are true. This could include:
a. CCTV footage
b. Check with people in the team in a discrete manner
c. Monitor chat messages from the computer
d. General observations on the floor, while supervising
If the evidence does come back positive, you can encourage employees or ex-employees to report the issue. The reason for investigating first, is to confirm the rumour and give confidence to the ex-employee to come forward and raise a complaint.
In the absence of a willing complainant, as an HR Person, you can also raise a complaint as a witness. The IC will need to investigate and give recommendations.
Hope you are able to turn the situation around quickly.