I agree it sounds like the air con. If you don't run the A/c all the time, you get mould smells from it. It's good practice to keep it on all the time to circulate the gas.
I had a leak a few weeks ago, the driver footwell was saturated! Rang my garage, and he immediately said he thought it was the windscreen seal, he'd had a few others to deal with recently, so I got it done. Dried out the footwell with one of those dehumidifier packs (from Halfords) which took a few weeks, but seems to have done the trick.
I've got another 'keen'... Ouch!