This achievement is on Case 3 and it is MISSABLE.
Near the beginning of Case 3 you interrogate Jayne Barret. She will give you her house security system records to back up her alibi. All you need to do is analyze these with the document analysis computer and find the discrepancy. This is the same computer you use to compare foot prints. Do this as soon as possible, just in case.
For a walkthrough of this case and a guaranteed way of getting this achievement please use this: walkthrough (Thanks to Yelll o for this great walkthrough)