This happens as soon as I take my eyes off you with 目を離す

Idiom Explanation: Averting one's attention with 目を離す

During a trip on a train, Conan accidentally overhears that there is a bomb on the train, and it’s set to go off. Conan tries to follow the criminals as they disembark, but Ran stops him at the exit door.

コナン:: 「あーーー」; "Ah...!"
蘭(らん):: 「もぉ！目(め)を離(はな)すとすぐこれなんだから！」; "Geez! As soon as I take my eyes off you, this happens!"