Since it's not against the rules and I was absolutely clueless as to where there might be another secret area I had a look at these levelmaps:
http://hol.abime.net/2223/gamemap
Found a new secret area each in the levels 4-4 (worth a lot of points), 7-2 and 7-3 (both few points worth though).
The one near the start of level 4-4 I would never have found on my own. I also lost quite a few lives trying to open its entrance.
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