"Believable?" Not so sure I'd go quite THAT far, but we oughta' be able to help a brother out.
1. A buddy was fooling around on his girlfriend and didn't want her to find out and
he left this in your office without your knowing, so you weren't even gonna take a chance of the boss or someone noticing it there and didn't want it in your trash can, so you slipped it into your pocket intending to toss it and just spaced it.
2. There were kids promoting safe sex in some kind of public service giveaway on a sidewalk downtown and this was a giveaway. You were curious about the brand since it wasn't familiar to you and you opened it and kinda got grossed out and tossed it but just spaced on tossing the wrapper.
3. You think you and your wife need to have a talk with the kids as you found it in the car and you grabbed it intending to show her and bring it up, but spaced it.
4. One of your carpool buddies, old college pal, co-workers, a guy from the bus stop/train station/BART/Metro etc. needed a lift and you think it must have slipped out of their bag/pocket/purse and you didn't want it in the car and you spaced tossing the wrapper.
5. [bottom line: blame a circumstance where you're the good guy...]
Last thought: if your office/weekly poker game/stupid buddy or some other setting having a gag gift of condoms as balloons works, try that.