The female bird was found hopping in the road in early June with its wings coated in an unknown substance.
Conservationists initially thought it had been caught in a glue trap but tests showed the oily substance was "partially digested fish matter".
The falcon was released into the wild again last week.
It was flown in a private plane from Guernsey for treatment at the Hawk Conservancy Trust's specialist bird of prey hospital in Andover, and back again.
Scientists analysed a sample of the bird's feathers and found that the substance coating them was a natural ester-based oil - which includes vegetable, seed and fish matter - with a high presence of sand and soil.
Ashley Smith, chief executive officer of the Hawk Conservancy Trust, said: "The results have presented a totally different scenario to the one first imagined when she was found.
"We now believe that the peregrine may have preyed upon seabirds, possibly a gull, which has used its primary defence mechanism of projectile vomiting to escape her.
"The partially-digested fish matter would have coated her feathers and she would then have tried to clean herself by dust bathing.
"This would explain the high presence of sand and soil in the sample, which had encrusted her feathers and prevented her from flying."
Paul Betchley, falconer with the trust who released the bird into the wild last week, said: "It was an amazing moment.
"We opened the door to the carrying-box and she hopped out, got her bearings and then took to the skies.
"Let's hope she's learned her lesson and will avoid gulls in future."
A peregrine falcon found injured in Guernsey and nursed back to health in Hampshire was probably left unable to fly after a seagull vomited over it.