But his return to his former club was pre-planned and during approved time off.
The 22-year-old striker raised eyebrows after he was filmed congratulating hat-trick hero Jose Luis Morales following a 3-1 La Liga win over Osasuna on Sunday.
On Saturday, Mauricio Pochettino had torn into his players, calling them "soft in every challenge" and "not ready to compete", questioned their "maturity, personality and character" and threatened to cancel two scheduled days off.
Pochettino also said: "We were talking in the dressing room after the game. We should go directly to the hotel and sleep and start to train early in the morning and not have a day off."
Sources have since confirmed that the players were allowed their time off, with Pochettino having only raised the idea of an early return to training as a possibility.
Players are free to travel overseas during their time off and it is common for them to attend sporting events in other countries.
Jackson is understood to be in Spain to deal with personal matters, having some loose ends to tie up relating to his move to England this summer.
He continues to move his life to west London as he adapts to the Premier League, and has scored seven goals in 15 games in all competitions since his £32million summer move.
The Senegal international watched the 'Yellow Submarine' for the first time at El Madrigal since he left in June. He scored 13 goals in 47 games across his two-year spell in La Liga.