Maintenance personnel found non-standard work on aircraft fuel tanks performed by maintenance vendor.

Narrative

When talking to vendor about work being done was made aware of faults after leaving station where aircraft X had much work accomplished including fuel tanks. Aircraft X had failed its test flight and then later pilots refused aircraft due to discrepancies between wing and FQIS (Fuel Quantity Indicating System) reading 7000lbs difference. Aircraft X was scheduled for maintenance in ZZZ where vendor found deficiencies in the work performed in the left wing wiring of fuel probes which were improperly installed not to specific Aircraft Maintenance Manual (AMM) routing and sealant work was in need of rework; left wing work was documented and corrected. Upon further troubleshooting left center tank was opened up and major debris was found; (absorbent mats; bottle; and metal funnel; along with sealant debris). This also was documented and corrected.Suggestion: Closer follow-up prior to closeup on fuel tanks by vendor and station inspection to follow General Maintenance Manual; AMM manuals for routing and general work performance. We were lucky that this did not cause an critical situation or crash as a result of fuel starvation.