I never found much of anything unusual in my trucks,except maybe a few roaches and a neat roach clip made from a cotter pin and a washer in one of my K5's...found some wheat head pennies in the ash tray and console in the same trucks too...my '71 GMC had a craftsman ratchet and 1/2"x 9/16" wrench in the front fender under the battery,and a 1/2" socket --all 3 looked brand new,but had likely been there a year or more..
In the junkyard though,we saw all kinds of strange stuff..some scary things too,like a fuse box in an old truck with glass fuses,that had a few .22 caliber bullets IN the fuse box!..

....one car,a 1950's Buick,had a huge Kodiak bear rug complete with the head stuffed in the trunk,which we left there,and it nearly gave many an unsuspecting parts hunter a heart attack,as it did us,the first time we opened the trunk up!..
One car someone bought just before the yard was going to be closed for good,a 71 Sattelite that was pretty well cannibalized and cut up,had been sitting in the same spot for over 10 years...a few weeks after it sold the guy came back looking for more parts and other cars to buy,and he showed us a photo he took of the inside of the rear 1/4 up next to the trunk lid...under a pile of duct tape,he found a a large chunk of hashish about the side of a deck of cards!..

...said it was too moldy to smoke though..what a bummer man!..
We'd often find lost joints and pipes in junked cars,and a few small bags of pot and some knotted baggies with coke in defroster duct hoses in a few cars that were hauled in for scrap after whoever was driving them got sent to the slammer..also found a lot of nice things in them like leather jackets,new tools,axes,machette's and hunting knives,and tons of mittens and gloves,some brand new...more than one junk car had the trunk piled full of practically new clothes for a toddler,we would dump them off at the Salvation Army box at a store up the road if they looked good enough to re-use...
One car had a full case of Sam Adams beer in the trunk,we didn't open it until one hot day we heard some "pops" in the trunk,and decided we'd better investigate...most of the bottles blew the caps off,the few that were intact tasted horrible after we cooled them in the fridge..probably years old!..