Running from max to min in less than 1000 miles i would have thought that shouldn't be to hard to find. But then that very easy for me to say sat here.
Signs of leaks can be a coloured stain. Blue, pink depending what colour antifreeze was used. Over a period of time as the fluid leaks you can get a build up of crystals around the leak. You might see steam.
Can you smell the coolent at all ?
Trust you have had very little change in the cars performance then ?
Any fluid under the car at all ?
You can replenish the cars system, however generally speaking it ain't going to get any better. Unless the leak crystallized and the leak seals it's self. Happen once on an old Vauxhall i used to have years ago.
Keep looking. Sounds odd i know but lets hope it gets a bit worse, should be easier to find then.
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Gordon.