What? Please elaborate on how to make the system guess.
I'll try to explain it simple. This is a feature added in ioquake3 and a very useful one.
This is called bash-style auto completion and can auto complete almost anything at all times except for callvote and kick. It will do partial completion or provide a list of possibilities if there are more than one choice.
I think your mistake is that you press enter or you are missing the space before pressing tab.
Bring console down: Shift+Esc
Write "/map"
Press space
Press Tab (a list will be shown but the engine cannot know what mapname you will write and cannot complete anything)
Write the letter 'f'
Press Tab again
Now if you have a 3rd party map starting with f then a list of maps starting with f will be shown if you only have "fan" the engine will auto complete the word "fan"