Minecraft 16w39c, Minecraft 16w40a
Mac OSX 10.11.2; Jave 8.101
The new observer block appears to detect the coupling of two tripwire hooks.
1) Place one tripwire hook.
2) Place observer block to monitor either the tripwire hook itself or the block the tripwire hook is attached to.
3) Set up a reporter system (redstone lamp in images below)
4) Place a second tripwire hook facing the first as if you were going to make a tripwire.
Notice that upon placing the second tripwire hook the observer block detects an update and turns on the lamp (see image). Note that no string has been placed. To demonstrate this, the second image show the same effect occurring with a solid block where string could have been. This works at larger distances than shown here, it will work with a maximum of 40 blocks separating the two hooks. It only works if they are facing each other and at the same Y coordinate. Note that the entire space between the two hooks can be filled with solid blocks.
No update is detected when the second tripwire hook is broken. Placing or removing blocks between the hooks also does not produce a signal, with the exception of placing or extending string that is attached to the observed tripwire hook.