This should be an obvious answer but humour me; are the pistons on genuine Peugeot shock absorbers (for a 6) supposed to rebound when compressed? If I compress the shock absorbers by hand and they don't return to their original height, that means they're broken, correct?

This is just to make sure that all shocks work in the same way that I think they do, and that Peugeot didn't make some strange suspension.

It should be noted that the shock absorbers don't leak or display any other untoward signs of wear.

they're filled with oil which moves from one chamber to another usually through a small hole, thus slowly controlling the rate at which the piston compresses. Oil has no ability to push the piston back out as oil doesn't compress. As Welshpug says, it's not gas filled, which does compress and able to return a compressed piston.

Have a look on youtube for some further information on how it all works.

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1999 GTi-6 (china blue)
1999 Rallye (black)