Tamper Evidence Bags Small, 190 X 260 Mm. Correctly packaged evidence is essential if reliable scientific results are to be obtained. Preserving the chain of custody is important because it maintains the integrity of the evidence. Whenever person comes in contact with piece of evidence, the contact must be documented to prove that the evidence was always accounted for and that no one tampered with it. These specially manufactured bags have many tamper-evident features, including unique bag number and printed label. These bags also have the added benefit of being leakproof.