Am not sure how ethical it is to film secretly but many docs and news organizations seem to do it. I wanted to have a camera or two that I could use just in case it was needed. Better to have the camera(s) in a pinch if needed then not to have them. It might one of those things I quickly grab out of my bag and stick in my pocket just in case it is needed during a nights shooting.
If a do a documentary in Thai next time showing the seedy side of the sex scene in Asia, either something on a worker in that world or on a sex tourist (thinking of trying a documentary on one of the sex tourist/sex pat lives). If I do that then a hidden camera is almost a must as those worlds are very secretive, filled with police corruption and some illegal activity (drugs, bribery etc).
I might look for a more expensive unit to test out, eBay is filled with such thing. Also hidden body microphones to record conversations.
Update* Just thought of another use, if the police in the border areas (both sides of the border) during the dump project are taking bribes or committing a beating or something else of a illegal nature, a small hidden camera might work better than overtly filming. It might be good idea to just carry one of these devices in case it is needed but never use it.