Ulterior

/ʌlˈtɪər.i.ər/ adjective medium

Existing beyond what is openly admitted or stated, especially a hidden motive.

His offer to help move furniture had an ulterior motive - he wanted a first look at the antique collection.

Memory Trick

Sounds like 'ultra-interior' - a motive buried deep in the interior, far past what is shown on the surface.