In Shrek 2, Puss In Boots says this of some catnip that's found on him.
Parodied on The Animal, where the main character catches someone at the airport using his newly heightened sense of smell. After he pulls a pack of cocaine out of the man's buttcrack, the man looks around nervously, and says "I don't know where that came from, but it's not mine."
In Harry Potter and The Chamber of Secrets, Argus Filch pretends that the Kwikspell course Harry found on his desk is actually for a friend, since it would mean that people would find out he didn't have magic.
South Park episode "Major Boobage". A cat is found in Kyle's drawer.
Then why did your mother find this in your dressing drawer?
All right. Look,that isn't mine. I'm just holding it for a friend.
Don't lie to us, Kyle! How long have you been on the cheese (huffing cat urine)?
In one episode of Family Guy, exotic toads become a trendy drug (so to speak) among high schoolers. Popular girl Connie D'Amico nearly gets caught with one at school, but hides it in Meg's backpack and tells her to hold onto it for a while. When Meg's parents find the toad, she tries to explain that she was just holding it for a friend - which, in this case, is mostly true (Connie isn't exactly Meg's friend).
In an episode of Futurama, Hermes' son Dwight has stolen what he thinks is a cigar from him when he's caught;
Hermes: That's not a cigar. [[[Beat]] and he realises] And it's not mine!
In an episode of Batman Beyond, Terry is caught with "slappers" (a performance-enhancing drug using Bane's old Super Serum). He truthfully tried to explain to his mom they weren't his.
"They're not mine! I found them!"
Transformers Animated: Starscream's compulsive liar clone has Sentinel Prime cornered, only for Optimus and co. to arrive.
Optimus: Step away from the Autobot!
Clone: What? This isn't my Autobot! I was just holding him for a friend.