I don't know if its location differs from canon to canon, but it's usually located on the roof of a GCPD station I think, so has anyone ever seen the signal, and just lain in wait to ambush Batman?
It's surprisingly hard to find examples of villains using the Bat-signal at all, considering how obvious of a target it is; even then, most examples are just baddies manipulating the signal to send Batman a message. However, there are a handful of times it's been used as an ambush point1:
In Batman Confidential #11, part of another version of the Joker's origin story, the Joker uses the newly-constructed Bat-signal to force a confrontation with his foe:
In Batman and Robin2, Poison Ivy uses a modified Bat-signal to attract Robin, intending to incapacitate him:
In the DCAU movie Batman: Mask of the Phantasm, city councillor Arther Reeves tries to set up an ambush with the Bat-signal, but Batman doesn't take the bait.
1 As I hint, I'm limiting my answer to occasions where someone is using the Bat-signal to lure a member of the Bat-family into a hazardous situation. There are more examples where the signal is used to communicate with Batman, either by a villain or a hero, but I'm not including those. I'm also not including occasions where individuals with non-villainous intent use the Bat-signal to get Batman's attention; I'm mostly throwing this in to exclude things like Nicole Kidman sex-ambushes
2 I apologize for reminding everyone that this movie existed.