2004 Vauxhall Corsa no water after being fixed

water pump broke, car nearly overheated but pulled over asap. Took it to local garage and they replaced the water pump! Drove 50 miles that night and everything seemed great, temp gauge was normal no noises etc.
Did a similar journey today and again evrything seemed fine. Went out to the car tonight to check my oil as i have a 2 hour hourney first thing and i noticed there was no water in the bottle again........i filled it with water then switche on the engine, immediately it turned red so i know that antifreeze was somewhere in the pipes as the bottle was completly empty prior to me re-filling.! Checked underneath for leaks...nothing and i looked at every pipe i could see and still nothing! Took it for a little spin and it seemed fine, heated up nicely and drove fine. Stopped to check level again and someone stopped and said it could be air lock.....is this likely as i have to drive over 200 mile in a few hours! I have water and antifreeze in the boot so i will keep checking evry so oftern but i dont wanna drive if it could be something else......also if it is an air lock will this solve itself?