Finally found this website that might provide the answer (since I saw this at Sycamore Landing).

http://parkplanning.nps.gov/projectHome.cfm?projectID=23312

I might have been misled because I was looking for another device across the trail to complement this, whereas this must be monitoring the entrance to the trail. The sensor on the device must be on the back side. The device is quite compact and must include a battery pack. I wonder why they screwed it to the tree rather than using straps.

And I confirmed the identity of the device a few weeks later.
http://www.pbase.com/kujoseph/image/155247517