I think a key that locks a door from crowbar or activation for say 30 seconds would be good, it would allow a traitor to escape.  The skeleton key could probably be kept in the lock after use, and the door not lockable again, but the detective can scan the door. Detective's could even have an item to unlock doors that have been keyed, so in god-forbid they have one on them whilst chasing the traitor, they could use it, or to get to C4. 
My other suggestion is the scanner 2.0. It could scan a dna sample, and whilst scanning more rapidly, it would have a disadvantage, perhaps the person being scanned being alerted they are being scanned, or the scanner not scanning accurately, and perhaps throwing the scan off by a few yards.

I was watching that latest episode of family guy, which was pretty lame.
Anyways, I had an idea for a one use item that would prevent DNA for one kill. Something like maybe 'The White Glove'. The knife would steal be there though, because the existence of a glove would not alter the existence of a knife? And perhaps when a detective scanned it, it would notify them of the glove.