Thanks Renovator. Is there any way to know for sure if that's what's happening?

Could be a number of things. Humidity, very high water table, type of concrete used, and lack of barrier when poured. Our basement is several inches thick and I think DH had a barrier put in, plus it has reinforced steel rods to keep it stable. Never leaked (so far), and we have a high water table.
How thick is the flooring? and how are the walls? Oh yes we have a drain tile around the house with very little vegetation.

Tape a 3 ft square piece of polyethylene to the floor with duct tape and pull it up in a week to see if any water is under it.

OK. I know polyethylene is plastic but I'm not exactly sure what you mean. Sorry. Can I just tape saran wrap on the floor?? Excuse my ignorance.

It can be anything that is waterproof and can be taped to the floor. Try a garbage bag.

Aluminum foil, a garbage beg, etc.

Saran wrap is not thick enough.

Before disturbing the taped down layer not if there is water on the exposed side of the layer.
Wiping gently with a single sheet of toilet paper works well.

After that pull up the layer gently and look for moisture on the down side of the layer, then on the actual surface.

Thanks! We're going to get everything off the floor today and then I'll try that. I appreciate the advice.

The chances are there is no vapor barrier under the concrete, nothing to stop rising damp up the foundation wall or impede vapour flow from the wall into the interior.

All of this is pretty much standard practice.

Running one or more adequately sized dehumidifiers is the solution in most cases.

To remove enough moisture to make a difference hook a large dehumidifier up to a hose that drains into a sink or sump pit. Set it so it doesn't have to run full time. It doesn't make much difference where it is located as long as it is not in a space closed off from the spaces you want to dehumidify.

Well, we started moving boxes and it was far worse than we suspected. Three rugs completely soaked and moldy, three boxes of books had to be thrown away, and some furniture damaged. Still, I can't help but think it could have been much worse if we hadn't noticed it until later.

Our landlord brought over a maintenance man to look at everything and he is replacing the sump pump and inspecting the drainage system. We now have three dehumidifiers running and all of our stuff is in new boxes and up on storage shelves.

Thanks again for your help.

Hi,

I had a similar mystery. I went into my basement and found about 1/8 inch of water just sitting there right in the middle of the floor. This was after heavy rain. The areas all around were dry. The mystery how did the water get there.

It turns out I had a small hole in the concrete wall where the cable TV cable comes in. Usually there is no problem but this year there was alot of rain. The water came through the hole and trickled across the floor forming the pool. The water trail from the hole to the pool then dried. Leaving the mysterious pool.

I suspect from your post that one of the basement walls has a crack. The leak will not show unless it is really wet outside, sufficient to raise the water table to where the crack is. The puddle of water forms and hence the soaked boxes, from the bottom upwards, in your basement. Over time the water dries, hence the mystery. The give away is the "soaked" shower curtain. I think if you have just condensation or seepage through the concrete floor, you will not get a soaked curtain. Dampness yes, but it looks like you had a puddle there like me.

best, Mike.

Concrete is 'waterproof' to the extent that water does not damage concrete.

It is NOT waterproof in the sense of not allowing water to pass through.

Concrete (and any Portland cement product) is extremely effective at wicking water through even many feet of material.

If the water is then trapped on the far side of the concrete it collects there, and any object paced on the concrete becomes wet.
Sometime to the point of saturation.

Even tile walls in showers have this issue.
The tile may be impervious to water passage, but the grout is not.
Neither is thin set used to fasten tile, or the cement board the tile is affixed onto.
A barrier is introduced into the system to stop moisture movement.

Classically it was behind the cement board, but there are newer systems that can be on the cement board and under the tile (membranes and applied barriers).

Some older mud bed jobs are thick enough to diffuse any water from the grout lines and do not have a vapor barrier (they are also many inches thick and expensive to install).