what was it?

[AirMoore]

Like AirMoore said, it could be any of a bunch of different things.

http://www.fastwrx.com/scanmaster3.html



A lot of different ancillary devices comes in a "box shape."

x2, There are just too many items to guess accurately, I wasn't even thinking along the lines of 'performance parts' Many turbo timers fit the description as well... again along with numerous other things.

[TSi+WRX]

^ +1.

FWIW, I agree with your initial response - that it was some kind of portable music device.