Does hard plastic (humidifier) develop micro-leaks?

I've had a humidifier and a cat drinking fountain both develop unexplained leaks. They both consist of a hard plastic base and a water reservoir that sits in the base, with a part that lets water into the base as the level drops. The humidifier has a second part which is a wick and a fan, the fountain has a pump to drop water into the open portion so the cats have fresh aerated water.
Each is about three years old. The humidifier was stored in the basement and when I put it together for this winter and put in a new wick, it leaks. I replaced the part that feeds the water with a new one, and it still leaks. I cannot figure out where it is leaking from.
The cat fountain was in continuous use until a few months ago when it started leaking. Again, I cannot figure out where the leak is coming from. It looks fine, and a few hours later a pool has formed on the floor. The water level looks no higher than normal.
All I can think of is that the hard plastic base(s) has somehow developed a permeability to water. Can that be?
I'm pretty mystified. I've replaced both humidifier and cat fountain (neither leaks) but I'm still curious.
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dgk wrote:

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Use food coloring, not dye! Food coloring will wash off or at least bleach out, dye is permanent (more or less).
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On Mon, 27 Dec 2010 19:33:26 -0800 (PST), "hr(bob) snipped-for-privacy@att.net"

I'm going to assume that the coloring will be evenly dispensed in the water that leaks out. I'm not sure how that helps me find the source of the leak since the water will still be on the floor.
But you did remind me of when I was a kid doing lifeguard duty on a small condominum pool. I learned from the pool supply company guys that if there was a leak we should put some oatmeal into the water and see where it ended up. I suppose I can put some oatmeal into the bottom and watch where it goes.
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If the leak is slow, you should be ale to see where it / water comes out before it makes a big mess.
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