Having just about convinced myself that I didn't have bed bugs, but rather a few carpet beetles and a bad dose of paranoia, I found two brown streaks along the underside seam of the sheet that lies between my mattress and the divan base. I'd add that I haven't smeared these myself, though as they are in a part of the bed that has a lot of weight on it and as I've been moving stuff around a lot lately, it's possible something else could have smeared them.
Here's a zoomed photo of one of them: https://flic.kr/p/AcH4JW
And an unzoomed of the other: https://flic.kr/p/zWivnJ