MacBook erased by thieves but still shows on FMM
To answer your actual questions, if they did not erase the hard drive and you did not encrypt it using FileVault, they could simply reboot in Recovery Mode and open a root terminal to change the password to your user account and have full access to the file system.

Regardless of whether or not you were using FileVault, I believe if they remove and re-insert the memory, it will unlock automatically.  Sorry for your loss. This is probably one of those times that it would benefit you to "know a guy," and by "guy," I mean a vigilante badass.   

It depends on your police jurisdiction (or more specifically, how helpful the cops are and how well you know them), but I have heard of friendly cops tracking down a stolen iPhone using Find My iPhone and getting it back to the rightful owner within the same night, but in most cases, they have bigger issues to worry about, especially if you live in a big city.