Officer Phil O'Neill did not consider the fact he could not see Daniel Adewole an emergency situation and returned 18 minutes later.
The 16-year-old, who had epilepsy, was later found unresponsive in his cell at Cookham Wood in Rochester.
Asked if he thought he should have gone back sooner, Mr O'Neill said: "I do."
Another officer, Sharon Bignell, told the inquest she had not unlocked Daniel's cell because the teenager "could have been hiding" and could have attacked her.
The inquest, being carried out by coroner Patricia Harding, continues.
A prison officer went for a cigarette when he could not find a teenager who was supposed to be in his cell, an inquest heard.