Translation:When I left the vacation home, I thought of the key.
Yes, it is in accusative here. Basically you use accusative here because your thought is directed at the key, and direction implies accusative while location implies dative. At least that's the way I would try to explain my use of the cases with dual prepositions as a native speaker. Perhaps this helps you a bit more: http://german.about.com/od/grammar/a/DualPrepositions.htm .
Der Schlüssel (or den Schlüssel if it's accusative) is singular, die Schlüssel is plural.