Five Welsh councils used undercover surveillance on staff
29 October 2013 Last updated at 21:32 GMT
BBC Wales has found that at least five local authorities in Wales say they have used private investigators to track staff or would consider doing so in future.
Gwynedd, Cardiff, Bridgend and Denbighshire councils said staff had been put under surveillance in "exceptional circumstances" such as when fraud may have been committed.
Caerphilly council stopped using undercover surveillance in April.
One teacher who was monitored while on sick leave said it had left her feeling vulnerable and violated.
Gwent Valleys reporter Paul Heaney reports.