I supported Navision 4.x years back and it contained a Change Log table that if enabled would log just about everything a user did in Navision. If that account showed within Navisions "user list" I believe it would log if deleted. I used this many times on users that stated "I did not do it".

Another place to check for Navision things is mibuso.com. I got a good bit of information out of this forum. You can also try a script someone published from that site for audit purposes as well: http://www.mibuso.com/dlinfo.asp?FileID=351