Ok, we spent one day running Techmuc's minimum script on a large variety on machines, several SED and engine versions, and with professional debugging tools. We could never see any memory leak, except for the mentioned leak in debug mode. We also had SED run overnight under remote control and permanently started that script. After about 10,000 runs, the memory allocation was still exactly the same.

So in short, we are not any wiser than before.

Maybe you're always running in debug mode?
Or still using something in your script that does not free its allocated memory?