Do pieces that come in used furniture count? More than 20 years ago one of my mom's close cousins and her family moved into a townhouse and didn't have room for everything, so they gave my parents a couch. Their son had a large Lego collection (lots of pirates, castle and space) and numerous random parts hitchhiked in deep in the recesses of that couch. It was almost like a treasure hunt for my younger brother and me when we found exotic items like a big trans-red antenna. More recently I was dumping bagged trash at the local dumpster and found about 30 pieces that didn't make it inside - mostly 1x1 and 1x2 bricks and plates plus a few other random parts. Too bad whoever threw it out didn't just drop the whole collection on the ground...

I work in a school, so I'm always finding Lego outside or in the halls (one of my students has a penchant for getting angry and throwing his Lego, it's great...). I generally leave it for whoever lost it to find unless it's somewhere that someone will fall on it. That's how I wound up with my favorite desk citizen: a sort of relative of Unikitty made out of one of those yellow 1x2 bricks with the face on them, a black 1x1 round plate, a blue 1x2 brick, a couple little slopes and other round plates. It's kind of really adorable.

I didn't get to rescue Emmett's hair before the janitor swept it up. :\

After buying my house a couple years ago, I replaced the washer/dryer. I found an old Forestmen minifig under the washer, in perfect condition.

I recently bought a cardboard box of misc LEGO parts and when I got home, I saw a few tiny pieces dropped. I traced this to inherent holes in the corners of the box, and found a trail of little pieces leading to the car. I can only imagine a similar trail from where I parked to get it up to their front door and inside their house. This had a good number of small technic pieces. :(

Pieces I have found over the years, that I can recall now:

Early 80s: 1 white 2x4 brick, dirty, found under the driver's seat of a used car my Dad bought minutes before. IIRC, my collection at that time was all of 1 set.

Late 80s: None, at least that I can recall.

Early 90s: 1 red 3x2 slope, on the street in front of my house, looked like it had been run over.

Mid 90s: many, including several 2xN bricks, a yellow 1x2 with Classic Space logo, 2 blue 1x12 Technic bricks, most of them stuck together in a wall form, all in the back of an abandoned Fond Off Road Dead pickup in the woods. About a year later, I sifted thru the rubbish in the back of said truck, and found a yellow 1x6 brick, and 1 (or was that 2 or 3?) yellow 3x1 inverted slope. Nothing else of value. Another year or so later, I found a blue 2x2 plate buried in the forest floor litter.

Later mid 90s: Found a yellow 1x2 tile w/ steering wheel in front of a Starbucks. Along a power line road south of my place, there used to be a load of illegal dumping going on, and I would look thru the trash, mainly for metal that I could sell as scrap, and occasionally found a few LEGO pieces, including a few bricks in a box of trash that smelled of solvent (meth lab trash?), the bricks lost the smell in a few weeks, a couch with the usual cache of bricks, and a dozen or more DUPLO blocks, which I cleaned up and gave to a niece.

Early 00s: A few bricks by a road up north, some had "runned over by a big fat car" damage.

October '04: A blue DUPLO 2x4 brick along Sunset Boulevard in LA, CA.

Mid '00s: A red DUPLO baseplate with a hole in it, along said power line road.

Spring '07: A red Pat Pend 2x4 brick in a back yard on Mercer Island, while working for the Ornamental Tree Guy. For some (sinful?) reason, it never occurred to me to ask the property owner about the brick's ownership. The property could have changed hands a dozen times since the brick was lost, but I would have disregarded that possibility as a valid excuse anyway.

Between then and now: None. Zilch. Zer0. Goose Egg.

Found three light bley 1x2 grille tiles (2412b) outside a thrift shop a day or two back. Also a 2x4 megablock. Didn't pick that one up.

When we bought our latest house, I found a minifig and an obviously small-child-built car in the loft.

I took it as a sign the house would be a good one.