Retmil has a good point. Not only could a crack in the hose he mentioned cause the problem but hose is cheap and easy to replace. I always go for the cheap and/or easy first because I usually replace everything while I am into a system anyway.

As odd as it seems the hose could have a crack and not leak coolant but when placed under a vacuum suck in air.

A system pressure test would help to find if there is a leak somewhere. Of course, everything expands when it is hot, but that is sort of built into the system. It is the increase in pressure that makes a leak appear.