Engine keeps using coolant

You need to have the system pressure tested

Here is a guide to help

https://www.2carpros.com/articles/car-is-leaking-coolant

I am having to add engine coolant or water every day but is not running hot. what is the problem? Why am I having to keep adding every day? I cannot find any visible leaks.

You will have to get a cooling system pressure test done to help locate any external leaks. do not allow coolant to run low as you will over heat and damage the engine. you may have to get the car on a hoist to locate any leak, it may be small and evaporating but if left unattended it will get worse, so act on this ASAP.

I have a 2001 Ford Escape with 81,000 miles on it. For some reason, it loses about 10 oz.s of water every 5 weeks or so. Enough to make the low coolant light go on. I tried changing the cap to the overflow tank with no luck. There are no leaks that I can see in the coolant system and no drips in the driveway. Also, no white smoke coming out of the tail pipe. There is the smell of hot coolant under the hood though which was why I changed the cap. No check engine lights or anything either. Both electric fans come on when it gets hot. Any ideas?

If you smell it, the coolant is probably burning off before you see a drip or leak. Look for traces of dried coolant around areas that got hot like the manifold.

you problably have an extern leak that evaporate before you can see it.
you may need to use a pressure tester to pump the system to the rated pressure while observing the external cooling system components for any signs of leak.

I have replaced the leaking coolant overflow tank in my 2002 ford escape but when I ran the engine to allow for the fluid to adjust there was no change in the level of coolant in the overflow tank. After letting the engine cool, I started it again and ran it with heater on high...the engine is still running hot.

Have you replaced the thermostat? Are the cooling fans working? Does it only overheat when it is in traffic or at highway speed too?