Not an answer regarding software.
However, before doing anything: is your companies' HR Department and legal counsel involved?
Depending on the company, workplace(s), local laws, labor agreements, etc. someone could be in serious trouble if the monitoring is not properly sanctioned and/or otherwise determined to be illegal.
If you are the one who will be implementing the covert monitoring then be very sure about the rules and requirements., Protect yourself - and even then you may vulnerable no matter what assurances you are "given".
I am not a lawyer (full disclosure) but spying on employees may end badly.