Hello, I hope the following assists you.
The cooling system is a sealed system that operates under pressure, coolant can't evaporate from the system without there being a fault. If the expansion tank cap seal were faulty you would have over heating concerns and usually the coolant would be blown from the cap.
Usually if the water pump leaks on this engine you can see a stain on the underneath of it, it is a common concern on this engine and as the garage suggests I would replace the timing belt and tensioner, the alternator belt usually requires replacement as well as this is a stretch belt and is cut to remove it.
The only other thing that could cause coolant loss without it being a visible leak is if the head gasket has failed and coolant is entering the cylinders, the coolant will burn and produce steam at the exhaust, it doesn't always leave coolant in the oil. A simple check with a block tester can confirm if this is the cause as the fluid in the tester changes colour if exhaust gases are in the coolant.
I hope this helps but if I can assist further let me know and I'll reply as soon as I'm able to.
1 No more Ford questions
2 Yes Good
3 No, your reply was on the ball.