It could easily be that there are additional checks because new releases include newer versions of the packages meteor depends on. Those packages are evolving and becoming more ecmascript standard compliant all of the time. There are many things in a compliant ecmascript implementation that are supposed to automatically turn on strict mode. See http://www.ecma-international.org/ecma-262/6.0/#sec-strict-mode-code. The list sounds pretty comprehensive.