I walk around the airplane--it's the one with the gouge under the wingtip. I can't miss it now that I know it's there. Everything is in order once I top up the oil, so we haul it out of the hangar and clear of the broken pavement FOD zone that has developed around the front of the hangar. I'm not sure whether the landlord or the airport authority are responsible for maintaining it, but they're not. We don't want to damage the propellers or the payload with flying debris, so ensure that we're on clean pavement for engine start and taxi. I call for fuel and then check all the caps and do my traditional last circumnavigation of the airplane before boarding. It's a last chance to spot an open inspection port, key left in a door, anything like that. I often wonder if other pilots will see me and think that's my whole walkaround, a literal walk once around the airplane. I spot something that shouldn't be there. I'm not even sure how I see it, but the mind learns what should be there and without even reporting to the consciousness when the eyes sees the expected, it quickly raises a flag for the unexpected.
I crawl back under the airplane. There's a screw head sticking out of one the main tires. More to the point, there's a screw point sticking into one of the main tires. The tire tread is good, not much wear, a big solid looking chunk of rubber. And the head of a screw sticking out of it. I have no idea how long this screw is. It's a pretty standard screw head that I've seen on lots of different length screws. It could be so short it's nothing. It could go right through to the tube. It could be only in the tire so far, but the force of one landing could drive it through to the tube and cause a blowout. I don't even know how long it has been there. Did I miss it on the proper walkaround this morning? The airplane could have been standing on it. Did I pick this up hauling the airplane out this morning or has it bee there for a week? Do I check every visible surface of every tire well enough every day I fly that I would have seen this as soon as it occurred?
The most likely case is that I can pull the screw out with my fingers or the edge of my knife blade and then fly the airplane with no ill effects. But I'm not going to do that because there's the possibility that the tire will go flat--either right away, or in flight while it's retracted. If anything is going to be damaged because of this, I want it to be done by someone who can repair it. I get an AME, who looks closely and pulls it out. It's a perfect ambiguous length such that it could still maybe, possibly have nicked the tube, but we don't think so and all agree that we'll go flying.
And the tire still hasn't gone flat, so we were right.