Ever think, "by golly, I'm really freakin' dumb?"
Thankfully, a Global Entry card is replaceable if for some reason you can't find it... but it will require a small fee for your own personal dumbassery. Straight from the source:
Request a replacement card by logging into your GOES account and selecting Manage Membership. Next select Request for Global Entry Card. A $25 fee is required for the replacement card. By requesting the replacement card, the original card will be deactivated and no longer functional!
There you have it. Maybe it's sitting where all of my business cards are?