It's much like the watchdog group that left Syria last year. They were sent to monitor violence, but it became too violent for them to stay in a warzone. Violence in an area of combat? Whodathunkit?
The results of testing (if ever truly known) would be interesting. Unless it's professionally manufactured, the shelf life of Sarin is not very long. We know it can be manufactured and implemented, however crudely, by small groups of determined people. The subway attack in Tokyo during the 90s was pulled off by a local death cult. Wouldn't surprise me at all to find out MB and AQ elements inside Syria were stocking up.