DAT is is a DATA file. It could contain anything as the file when complete could be a program, mp3, picture etc.
You can sometimes open them with notepad. Also Windows media player, weather or not you'll be able to see, hear what the file is varies...
"on checking my comp after she left, on my cdrive ,in my download,s found 3 of these file,s "2,804kb, 402kb, 1,0004"
Delete them if you have no use for them.
You could aslo ask her?