I'll chime in as I was previously an "air frame structure tech". Basically I did everything on the airplane other than mess with engines or electronics. Worked on the Navy P3 Orion fleet (remove and replace wings), the Airforce C-17 fleet (worked various tech bulletins as well as air frame inspection checks), and a brief stent on the the 747 'Sofia' project (cut a really giant hole for a telescope)

Anyway, I was most intimate with the P-3 orion fuel tanks (which is the entire wing) as we regularly pulled the wings on those planes, replaced various spars, ribs, etc and then reinstalled the wings. Then we'd have to fill the entire wings up with the 'anti explosion foam' which is in several hundred cube shapes, all with painted on numbers (yes they all go back in order), ranging from 6" to 18" in size. After that was done, plane was refueled and tested. Normally after a few weeks of checks, the fuel filters and intake strainers would be checked. They commonly had rags in them, but always had tons of metal shavings and wads of human hair (probably hair from your legs).

So yes, 'FOD' is always in the tanks after any construction or maintenance. There would be no tools in the tanks as normally your employee number or social security number was etched on everything you owned.