A pair of black knickers appeared (clean) in a pile of clothing I had ready to wash in the living room. They are not mine. 
I asked DP and he genuinely seems clueless. We work together + we spend 95% of our time together. I do work 2 evenings a week, but during this time DP is looking after our toddler & I'm back within 2 hours of DS going to bed.
We haven't had any female guests staying. DPs male friend stayed with us 4 weeks ago and it did cross my mind, but they'd be far too small (size 6) for him anyway.
Anyone want to help me play detective?