BTW to the OP - it is the intermittent part that is confusing (to me) - but I thought of something thats a long-shot.
My skimmer has a float switch the turns off the air - if the cup is nearly filled - sometimes - since I tend to use more wet skimming, the basket fills faster than the tube leading to the 'catcher' can't drain fast enough - and it can sometimes cause a brief odor.
But - since your dehumidifier is only a week old - I cant see that being the problem so - Are you emptying often the bucket that holds the water? I guess a little more diagnosis:
1. Did you ever smell the smell before the dehumidifier was put in
2. Does the smell get worse - or better when the dehumidifier is on.
3. If you had one dead snail - you could have more - but that should not cause an intermittent odor.
4. Is the odor stronger near 'anything' - i.e. the tank, where the dehumidifier is, carpeting, etc. Why - out of curiosity did you get the dehumidifier