In my mind, the most likely thing they were doing was trying to get fuel out of the tank rather than putting anything in. Given the increase in fuel prices, this has become a problem. So, if you found no evidence of foreign substance in the tank you might consider the possibiity that your problem is not associated with the fuel door being open or at least that it is not caused by a foreign substance.
How much fuel did the mechanic find when he drained the tank? If the tank was drained, you may have sucked some air into the system and it could be the source of your problems.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13