Or give the target a Charisma Save... if successful immune to being detected for 25 hr.
Nah. No need for houserules. I'd rather see the LG paladin squirm:
Paladin: *Well, he is the mayor and lawful representative of the town, but undead are evil, or at least, I think they are always evil. Shoot, I should have paid more attention back at the temple. I thought that everything was evil that set off my Spidee Sense. But why did he get voted in as mayor. Is the entire town evil? Darn, I wish I had an Int higher than 9. Now the wizard will make jokes about me if I bring this up later.*
"AAATTTAAACCCKKK"!!!
Rest of party: "What the?"