ESC'ing from Mission always shuts down COD

[DoolittleRaider]

I am having trouble with Steam ...when I exit a Mission using the ESC key, the COD game shuts down and flips me immediately to the Steam account Library, where I have to re-start COD from scratch.
...
Any advice on what i may have done wrong will be appreciated.

Ohhh..also...this situation appears to be the same whether I have indicated on Steam that I will be offline, or connected to internet.
[By the Way, I am strictly an OFFline CODder, if that matters. Never multiplay.]