Sure, I think that a few ISIS members are probably hiding among Syrian refugees. However, I believe that ISIS is also pretty much everywhere. I'm sure plenty of their members are also United States citizens. Stopping refugees from entering our country to start a new life is not going to stop ISIS.
There is no way to tell right away when anybody is part of a group, this is a very good reason to believe that Isis members are hiding and waiting among refugees and among citizens in all countries. Think of it as a bowl of 1000 M&Ms there are 10 poisoned, would you grab a handful and stuff it in your mouth hoping you dont get a poisoned piece? Since Isis is known to be predominant among muslim communities, these people should be subject to additional screening, especially if they come as refugees from an affected country.
Of course ISIS operatives are hiding in amongst Syrian, and all other, refugee groups. It is just basic operational strategy to use any means necessary to gain access to your enemy. That is no reason to deny refugees asylum in other countries. The presence of ISIS operatives is just a cost of doing the humanitarian thing,
Syrian refugees are under a great deal of scrutiny. They are requied to provde a great deal of documentation to prove they are who they say they are, and they undergo lots of questioning and a strict background check. It would be very hard for members of ISIS to hide with refugees.