Your truck probably has a standard transmission, right? I've seen similar things a small handful of times, always with a standard transmission. I believe that these are from a throwout bearing that flew apart from not being replaced soon enough. If it gets bad enough for them to actually fall out when the bearing falls apart, the spinning clutch and flywheel can throw them around inside the bellhousing. Usually, I've seen these stuck on the lowerpart of the bellhousing and they are stuck in a mixture on grease and clutch dust. I've seen them stuck on the inspection cover in the same goo a couple of times. If everything was working fine before pulling the engine, I wouldn't worry about them. Some of the trucks I've worked on weren't very well maintained to be REALLY nice about it. Truthfully, some were a real POS, to be not quite so nice.