Advise pls

[Ghostami]

Hi Guys, forgive me for the long story that follows :

 

1) Car goes workshop for some overhauling for 3 days

2) Car returns to owner, from what I know, battery out of car for 3 days

3) Headunit now works with no sound produced

4) Checked fuse and all arn't broken

5) Check voltage supplied to amps, all showed 12v.

 

Before I took my car home, I highlighted the problem but its a Saturday night and its quite late, hence I took it back and tolerate for it for the moment before going back on Monday. I heard from one of the mech that some headunit need to do something to it to have sound if you take out the battery for too long, is this true? Mine is a pioneer headunit.