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.

The men aged 38, 42, 45 and 46, were arrested after a car was stopped in the Ringfort Road area, in Hazelbank, on Thursday night.
They were questioned at Antrim police station.
Police said their enquiries were continuing.

It was Guiseley's first league victory since 30 April - and the manner of it was enough to see the club trending on Twitter.
The game featured seven goals, a 37-minute delay when the lights went out and an angry fan marching on to the pitch to confront York City boss Jackie McNamara.
York, relegated from League Two last season and backed by more than 700 fans for the away trip, dropped to 20th with a fourth straight defeat.
Guiseley remain bottom with six points from 14 games.
The visitors led through Matt Fry's header, only for Guiseley to scores twice in three minutes through debutant Jake Cassidy and Alex Purver.
Purver added a third, Jordan Preston made it 4-1 and, after the floodlight failure plunged the ground into darkness, Jermaine Hylton, another debutant, and Rob Atkinson completed the rout.
The game, which started at 19:45 BST, finished at 22:35.
Match ends, Guiseley 6, York City 1.
Second Half ends, Guiseley 6, York City 1.
Goal!  Guiseley 6, York City 1. Robert Atkinson (Guiseley).
Substitution, Guiseley. Adam Boyes replaces Jermaine Hylton.
Substitution, Guiseley. Michael Rankine replaces Jake Cassidy.
Substitution, Guiseley. Nicky Clee replaces Connor Brown.
Substitution, York City. Kaine Felix replaces Justin Johnson.
Goal!  Guiseley 5, York City 1. Jermaine Hylton (Guiseley).
Substitution, York City. Fraser Murdoch replaces Scott Fenwick.
Substitution, York City. Yan Klukowski replaces Ben Clappison.
Second Half begins Guiseley 4, York City 1.
First Half ends, Guiseley 4, York City 1.
Goal!  Guiseley 4, York City 1. Jordan Preston (Guiseley).
Jordan Preston (Guiseley) is shown the yellow card.
Goal!  Guiseley 3, York City 1. Alex Purver (Guiseley).
Goal!  Guiseley 2, York City 1. Alex Purver (Guiseley).
Goal!  Guiseley 1, York City 1. Jake Cassidy (Guiseley).
Goal!  Guiseley 0, York City 1. Matt Fry (York City).
First Half begins.
Lineups are announced and players are warming up.

Russell Sherwood, 69, of Cilfrew, Neath, was heading for Ewenny in Vale of Glamorgan as Storm Angus hit in the morning but never arrived.
He disappeared at Stormy Down and registration plates from his car were found on Monday.
South Wales Police said it would try to recover the car on Wednesday morning.
Officers will remain at the scene overnight and the force said Mr Sherwood's family were being kept informed of any developments.
Mr Sherwood left his home in a silver Ford Focus at about 06:15 GMT as flooding caused by Storm Angus hit roads in the area around Stormy Down.
Tuesday's search focused around the dipping bridge on the river, and firefighters and coastguard teams were involved.
The search was stood down for the evening at about 16:30 and will resume at first light on Wednesday.
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.
[NEXT_CONCEPT]
Four men arrested in Londonderry by police investigating dissident republican activity are released unconditionally.
[NEXT_CONCEPT]
National League bottom club Guiseley came from behind - and overcame a floodlight failure - to thrash Yorkshire neighbours York City at Nethermoor.
[NEXT_CONCEPT]
A car thought to belong to a man who went missing in Bridgend on Sunday has been found in the River Ogmore.