Press Association Sport understands the 25-year-old United States international was booked on a flight to Canada after travelling down to London from the north-east on Wednesday evening with Defoe, 32, having headed in the opposite direction.
Altidore's proposed return to Major League Soccer to replace the former Tottenham frontman in a player-plus-cash deal remains a major factor in Defoe's switch to the Black Cats, but the move will have to be ratified by American game's governing body
MLS operates an allocation policy, meaning Toronto FC do not necessarily have first refusal.
Head coach Gus Poyet, who worked briefly with Defoe during his time on the staff at White Hart Lane, will keep his fingers crossed as he attempts to re-shape a strike-force which has proved less than lethal to date this season.
Of the 18 goals the Black Cats have scored in 21 Barclays Premier League games to date this season, only six of them have come from their frontmen with Steven Fletcher managing four, Connor Wickham two and Altidore none.
Indeed, Altidore has scored just a single league goal during his 18 months on Wearside and while his chances have at times been limited, he has never been able to reproduce the kind of form which brought him to Sunderland's attention during his spell in Holland with AZ Alkmaar.
However, while Sunderland have tempted former England international Defoe back home and out of his native London with a lucrative pay packet, it is understood Altidore too will cash in if he agrees a switch back to his own country.
Source : PA
Source: PA