if you just open a console it will be in /root/file.cap or file.ivs
if you use airoscript will be in /home/root
if you use one of my scripts it's always saved in a directory named with APMAC under the script directory (usually /usr:bin/scriptname/)
in console the file is in the directory from where you launched the command.........
or you can specifie a path .....
airodump-ng -c 10 -w /im/a/wannabee/wepcracker/capture rausb0
will create the file capture-01.cap in /im/a/wannabee/wepcracker/ folder