A thread on the Steve Jackson Games Forums about using Glamor or other means to magically tamper with or remove evidence from a crime scene.
There are a lot of ways to go about this.
Obscure (Forensics) would give a penalty to the skill to analyze the crime scene. However it has a duration and though that can be bought up it adds cost and is more suited to illusions.
Telekinesis, especially with One Skill only is a good way to alter and especially move evidence.
Of course you can also destroy evidence with Innate Attack, though that is likely to be messy and just call more attention to things!
Payload is good for sneaking evidence offsite or carrying things in to frame someone else.
Ultimately my choice is Control from GURPS Powers.
Control (Evidence) is a very broad category so fair at 40/level, making it rather pricey. However that would let you manipulate solids, liquids, and gases and even a single level would handle most evidence. Even minor changes could ruin evidence for practical use, remove fingerprints, reshape bullets to hide ballistics, move chemical traces, etc. Alternatively, instead of just damaging things beyond useability or moving material the evidence could be altered to frame someone else or make it look more like an accident.. This should definitely require skills and knowledge to do but the user could change fingerprints, writing, DNA, etc. to try and match another sample if desired.
Create (Evidence) with Destruction is another way to go and fits a spell or power that simply removes evidence.
Another variant would be a cheaper version of Control (Evidence) [5/level] that required a contest of skills or a per level penalty. The tradeoff of an absolute for a minor penalty justifies a much cheaper cost.