つまみ食い

彼（かれ）はケーキをつまみ食（ぐ）いするのを見（み）られた。

He was seen to eat a piece of cake secretly.

つまみ食（ぐ）いと言（い）うか、完食（かんしょく）していたように見（み）えたが。

You say you had a nibble, it looked to me like you'd polished it off.