Kay, here's my two centicreds:

Option A is best for trivial pieces of out-of-place or otherwise suspicious evidence.

Option B is better for suspect evidence which is key to the story being told...if the PCs requested(or performed)labwork finds nothing during the opening scenes, perhaps a later analysis of the evidence starts to reveal that something is more than a bit off....

Option C? Cryptic notes/comp entries/comms stumbled upon during the course of an investigation might lead the PCs to the possibility that some or all of their evidence is tainted. 

Or maybe the NPC lab tech who performed the initial analysis on the suspect evidence is later accidentally brutally murdered...or commits suicide and the note left at the scene points the way(what, your suicide didn't leave a note?! Suicides always leave a note).

Or...maybe just before the PCs find the suspect evidence, one of the NPC techs acts a little suspiciously at the crime scene....

Another possibility, a variation of option B...the PCs have the right being in custody, but, in a nice-Law And Order-style twist, it is revealed during the course of the trial, that the key evidence on which the prosecution's case is based turns out to have been planted at the scene of the crime or on the suspect at the time of his arrest, forcing the players to madly scramble about to find a)who, how and why that evidence was planted and b) some other compelling piece of evidence that ensures the conviction of the bad guy.

All before the judge issues his decision to dismiss the case at 10:00:00 GST the next day.