My son breadboarded a circuit that uses a white LED and is powered by a bench supply that internally uses a switcher. He came running to me to ask why the LED remains dimly light even after releasing a button he hooked up to control the LED. While demonstrating it to me, and failing to get it to malfunction the way he described, I discovered it was because sometimes he'd touch one of the LED leads, which would make it light up very dimly. Even with one of the power connections from the power supply disconnected, it would still do it.
I thought this was interesting, but not completely unexpected. We all know what happens when you touch the input of an audio amp, scope, or a logic probe (remember those?). So clearly there's some voltage there. I got out the current meter and held one probe while touching the other to the LED and saw around 800nA on AC mode. Sidenote: wow aren't modern LEDs efficient!!
I've never give this phenomenon much thought in all my decades of tinkering, but what is actually going on here? First I propose there's some leakage between the power supply output and one or more of the AC conductors, although this still occurs to a lesser extent when the supply's hard power switch is off, thus breaking the hot/line. But are we picking up AC leakage or fields and returning to earth through the power cord? This was in his shag carpet bedroom so I'd assume that's not very conductive.