Your lady friend may have stopped wondering about you, but now I sure am!
My Junior year of high school; I along with several classmates won our state competition in a consitution challenge deal- and went to Washington, D.C to compete nationally. (We failed miserably- but it was all about the trip anyway 
)
On the way back, one of the girls had a dress that she wasn’t able to fit into her bag… she turned to me and asked me if I had room in my bag. I said fine, tossed it in there and we both totally forgot about it.
That is, until I came home from school a few days later- and my mom confronted me on why I had a dress amongst my clothes. I gave her my truthful explenation, however, to this day I am pretty sure she is less than convinced.
Especially considering:
It must have been… oh… six months later or so, we were moving some furniture around in my room. Suspiciously shoved under my dresser was a pair of womens underwear. Making it worse- I couldn’t figure out how in the world they’d gotten there.
So- while my mom and dad stared at me asking me who’s underwear they found under the dresser. I stammered and hemmed and hawed as I tried to figure out myself- How in the world did a pair of girls underwear get under my dresser, and who did they belong to? Generally- I stood there looking guilty and I imagine- looking like I was trying to come up with a believable lie on the spot.
Believe me- there was certainly no action going on in my teenage bedroom, Socially Backward Nerd that I was. My mother pipes up that either I’m cross dressing or else “something else is going on…” what with the dress and now the underwear. Well, I mean- unless you actually are a cross dresser, which is the preferrable option for you? Especially since I was truely at a loss to explain the mystery underwear. So I got an impromptu talk about safe sex and responsibility and so on… and my parents now believed I was big pimpin’.
It wasn’t until months after this event- that it finally hit me. At my sisters wedding who knows how long before the “Mystery Underwear Occurance”- my Aunt and Uncle had stayed in my room. Surely they belonged to my Aunt.
Ahh well. At least I got to live vicariously through my parents imagination.
Whose bra is this
That’s made to rest
On someone’s breasts, supportingly
Whose cups would be but half full
If worn by my teenage daughter
Denial is the thing!
'It isn’t mine! I know it not!
‘It must belong to her
Whence else would come such a thing?’
The closest I’ve come to the OP situation was once when I found a ladies bracelet in the parking lot. It was broken in such a way that it could be snapped back together but would need glue to be permanently repaired. I dropped it into the console in my car in it’s reassembled condition. And promptly forgot about it. About a week later my then-wife found it while looking for something. Fortunately, being able to demonstrate it being broken made my found it story credible.
Flashback. Umpteen and a half years ago when I still went to clubs. Some nights when I was really bored I’d go into the restroom and get 6 or 8 condoms from the wall dispenser. Then I’d wander around the bar area leaving them in random gapped open purses or jacket pockets and tucked into hat bands. (Cowboy hats have a belt on the inside that’s convenient for storing small items.) I liked to think that led to some interesting conversations later on.
the one single long straight black hair in the shower…
found by a blonde now ex girlfriend…
she wasn’t even wearing her contacts…
damn!
I did that to my poor father once. As a pre-teen, I used to spend each weekend at his one bedroom apartment, and I slept on his waterbed while he took the couch. I was only about a B cup at the time, and didn’t always notice if I wasn’t wearing a bra, as I was new to it. One night he had a woman over, and she found one of my bras tucked into the side of the bed.
Johnny L.A., you bastard, I was just about to do that! Damn you! 
I held off as long as I could! I did succeed in not posting it yesterday. But in the end I just couldn’t resist.
Bras schmas. Those can be more easily explained away. Try explaining away panties. Yes, it’s happened (more than once) - but I have a theory. I think women hide them purposely to mark their territory or cause trouble for the next gal.
Why can’t they just pee around the perimeter of the room like everyone else?! 
I can imagine the look on her face when he said “It’s my daughter’s!”