why does the coolant in my expansion tank not seem to expand when water is hot ?

[Fastlife91]

I replaced the expansion tank in my car, the replacement wasn't new as it was a several days wait for a new one, so I bought a used one and put that on.

When I look at the coolant from hot it does not seem to be any higher than it is when cold, in fact sometimes the level looks to be lower when coolant is hot than it does when cold ??

 

When I check coolant from cold it often look higher level than when it is hot. I notice lately that after several days, I check engine coolant and it seems to be dropping a bit each time.

 

could the replacement tank be faulty, any other causes ?

 

 

edit: no leaks visible, already replaced hoses last time it was leaking, I paid for a head gasket job about 9 months ago and that was just days after buying car as garage said it was blown, later found out they lied and was just water pump but had no evidence so didn't confront them.

Car runs fine (perhaps but not sure a slight lack of power but put  that to faulty MAF) , no smoke whatsoever, no contaminated oil, e.t..c