Aircraft maintenance technician reported FOD inside fuel tank.

Narrative

ZZZ [Airport] field service aircraft XA41 Day 0. On Day 0 technicians was awarded emergency field service to ZZZ for aircraft 757-200 left wing dry bay leak. When tank access panel was opened a large piece of FOD fell out of hole. Upon further inspection of left fuel tank boost pump pickup screen area more FOD was found; appeared to be absorbent blankets totaling 4ea with a dimension of approx 32' in length and 14' wide. All FOD was removed from tank area. After a discussion with maintenance control it was determined that an inspection of [center] and [right] main tank be accomplished. Upon opening [center] tank access panel inspection of that bay revealed tooling left behind; Some type of pick approx 4' long. Inspection of right main tank did not reveal any FOD and boost pump screen pickup area clean. I would like to add that I have been a technician for over 15 years and over the years have found many items left behind by previous tank entries; but by far this discovery was one of the worst findings. This type of maintenance on our fleet is unacceptable and very concerning not only to me but all of us on the tank team. Thankfully this was found before the boost pump pickup screens became blocked resulting in engine flameout while operation under ETOPS.Suggestion: If this was the result of a third party contractor or vendor they too should be held to the same quality control standards that all technicians are held to. These clearance to close protocols are in place for this very reason and if not followed could result in a much worse outcome.