had you just washed the cake saver or anything like that? is the cake iced with fondant or buttercream?
i've had condensation on the inside of my cake caddy when storing a buttercream cake, i just took the dome off for a couple of hours to allow the cake to dry off and made sure to dry the dome out really good before putting it back on. everything still looked fine.
It has happen to me when I have tried to cover a cold cake with fondant, but not at room temp. That is weird 
Let us know how it worked out.
This is not a fairy tale but; Once I had a cake that I tried to pour fondant over and it started to harden as soon as it hit the middle of the cake. I could not even scrape it off! So the cake had a inch thick of icing...ugh.
I dont think you want your cake to air tight. I had some experience with it getting too humid. Try just putting your lid on and stick knife or something under the lid to allow some circulation
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