This morning there was a very wet spot right in front of my bathroom door. We live in a single story house with a slab foundation. I checked outside and found some water dripping from the sliding. We checked the copper pipes under the sink (between the walls) and found no drips or leaks . So what do we check next?

wet spot right in front of my bathroom door

Assuming that this is not next to a wall that could have pipes in it. With no one using any water anywhere, go out side and look at you water meter. Does meter show that water is being used? If it does, go inside and turn off the main shut off valve. Look at the meter again. Does it show that water is being used? If it does not, you have a leaking pipe underneath the slab.

Let's see if you have a pressure or drainage leak. To check for a pressure leak on city water, shut off all water usage in the house and go out to the water meter. Check the little pointer. It should be absolutely still, no creep whatsoever. If it's moving then you have a leak.
If you're on a pump then make sure the pump's up to pressure and shut off water usage in the house. Turn the pump off at the breaker box and watch the pressure gage. If it falls back steady then you have a pressure leak. Let me know what you find, Good luck, Tom