Trip an alarm...

Lex: They don't think I'm the first one that's been hit. There have been over a dozen of these
robberies all over the county. This gang somehow gets in and out without busting a single
lock. They even tried to hit the Smallville Savings and Loan.
Clark: I didn't hear about that.
Lex: Banks don't publicize attempted robberies, Clark. It's bad for their image.
Clark: If nothing was taken, how do they know they were robbed?
Lex: Apparently they tripped an alarm (triggered?) inside the vault.
Although no one knows how they got in there in the first place.