Just through a monkey wrench in the engine, or leave it on an inlet cowling.
Ok....so, did they throw the wrench in prior to engine start or after start? If they threw a wrench in, it would cause the damage on start, and it would be significant enough that the pilots would notice it. If they left it on the cowling, it would be detected by the walk-around...Oh..wait..no..the pilots were in on it! They saw the wrench waiting to be ingested on start, and left it there. There was a time release glue holding it in place, and they knew that they had to delay the flight an hour so that the glue would let go just after take-off. Sorry. That doesn't work.
You're right. It was a stinger. They shot it down. That's all it could be...couldn't possibly be mechanical failure on an engine that was just short of having a major overhaul.